Why Specialize? There are both pros and cons to specializing in one form of poker. For me, the pros outweigh the cons.
"Jack of all trades, master of none"
If you attempt to become good at a lot of different things, you'll never reach your potential at any of them. Assuming your goal is to increase your hourly earnings, I recommend you pick one form of poker and work your butt off to become the best you can be at it. You might decide at some point to switch games if you think you can do better elsewhere. Until then, master one niche if you're in it for the money.
"But I'll never become a complete poker player if I play the same thing all the time."
That's true. In the past (before online poker), being competent at different games was important, because game selection at the particular cardroom you were currently standing in might be sparse. If you were a limit hold'em specialist, there might be only one game going at a stake level that interested you, and if that one game had no value (meaning little or no bad players), you either had to play in a bad game or not play at all. But if there was a juicy 7-card stud game going, and you were competent at that game, you would be back in business.
But online poker players need not worry about this. There are always plenty of juicy limit hold'em games going. There are even almost always plenty of juicy Super Turbo SNGs running on Full Tilt - and that's a pretty small niche!
The one exception I can think of where you might have a hard time finding a game is if you are someone who plays very high stakes cash games. Because of the small player pool, high stakes pros often feel compelled to play no-limit hold'em 6-max and heads-up as well as pot-limit omaha or a mixed game. But if you're playing at the intermediate levels or below (where games are abundant), I think you should be focused on mastering some form of poker. You can always switch later - just don't hop back and forth if you are trying to maximize your earnings. The better you get, the sooner you can move to higher stakes and/or play more tables competently - thus increasing your hourly earnings.
Many online poker players like to dabble in different games or in different forms of the same game. Maybe their main game is 6-max no-limit hold'em cash games, but often they'll find an excuse to play heads-up or full ring or even a different game such as pot-limit omaha or MTTs or SNGs. They tell themselves things such as:
"I am afraid I'll get burned out"
This can happen, but I suspect most burnout is caused by not having a big enough edge. Players with a tiny edge are constantly stressed out, because of losing months or big downswings. THAT's what is burning them out. And that's why it's important to work hard to have a big edge on your opponents. If you're consistently winning an amount of money that is meaningful to you, you won't get burned out. You might become physically, mentally or emotionally drained, but as soon as you rest up, you'll be eager to get right back to making money.
But if you legitimately feel yourself getting burned out, don't expect to recover by doing something that's only slightly different. The best way to recover is to get away from poker for a few days. Even consider getting away from your computer for a while.
Afterwards, if you're still having trouble, then consider switching specialties (at least temporarily).
Many successful online poker players practice serial specialization. They will play one specific game for a few weeks or months then switch to another game for a while. They might do this either because they are getting burned out or bored or maybe they think they could make more money by switching. The difference between serial specialists and dabblers is that dabblers spend a smaller percentage of their playing time in the "flow state" and very few dabblers ever get really good at any game.
The Flow State
It might seem obvious why playing just one form of poker would allow you to achieve a bigger edge, but the reason for it goes deeper than you might realize. When you jump back and forth between hold'em and omaha or SNGs and MTTs or even regular SNGs and Super Turbo SNGs, it's harder to play your A-game because it takes longer to get into the zone or flow state.
Flow is the mental state of operation in which the person is fully immersed in what he or she is doing by a feeling of energized focus, full involvement, and success in the process of the activity. Colloquial terms for this or similar mental states include: to be "on the ball", "in the zone", or "in the groove".
If you are playing different games from day-to-day (or simultaneously), you'll be using a lot of "mental RAM" thinking about basic aspects of play. It will take you longer to get into the zone (if you get there at all). And you'll never reach the same level of energized focus that is achieved by a specialist. This will make you less efficient and slow your long-term progress (of moving to higher stakes and/or playing more tables), keeping your hourly rate lower than it could be.
Specialists seem to have a sixth sense about what is going on in "their game", and they quickly reach the mental flow state. They are able to read their opponents hands more accurately. They pick up on timing tells better than everyone else. They get subconscious clues from other players' whinings in the chat box. They anticipate the upcoming changing conditions due to the blind increases of their particular SNG structure and instinctively know when to make
"Negative EV Plays".
They confidently and quickly know what to do in any situation. They are harder to trap. They know every trick and when to use them. They are aware of their image on each table and know how to adjust if necessary. They are mindful of every aspect of their game - they are masters of their niche. And they do most of this with seemingly no thought - it's automatic.
This isn't the case for jacks-of-all-trades.
Any player that wanders into the domain of a specialist is at a disadvantage - even if that player is an online pro in a similar but different niche. It's like a bug flying in a spider web.
Try looking up the Super Turbo SNG results for known online pros whose main game is something other than Super Turbos. They are almost all big losers. Likewise most SNG or MTT pros struggle in cash games (at least in the beginning).
So don't let the cliches about "not being a complete player" or "burnout" or "maybe one day NL Hold'em won't be popular any more" hold you back. If you want to increase your hourly earnings, you should specialize.
Becoming a successful specialist takes more than simply sticking to one game and playing a lot. You must work on your game away from the tables. "Deliberate practice" is the only path to greatness in any game.
Here is an example of how to apply it to poker.
Deliberate Practice for Poker
go from "specialize" to "Ways to Increase Your ROI"