I often wonder about this, I cant play more than 4 tables at once (and quit doing that as it was no fun) but I havent ever ventured into the world of Hems and HUDs and such. But do people play 24 table very low stakes to get a better hand history to see leaks but as one person said, they kept timing out on a lot of tables, so is playing that many tables reduce (I DONT KNOW WHAT IM SAYING HERE) why not just play $10/$20 tables as opposed to 25 microtables. Some of those big pots seems would equal a lot more than taking 25 microtables at one time. If yo have 20 tables running, with min $4 per table so you have $80 already floating out there. Why not just buyin to one table with $80 where you can really focus on just that game, or is my thinking just wack
To answer your question, playing more tables at lower stakes actually decreases variance. Lets say you play 20 $4 tourneys, you will cash at X rate, you will likely cash at a lower rate at the $80 table as the competition is better. So you have more chances to win in the $4 game, but also the reward isn't a great, but overall, variance should be lower.
Each to their own, you can say that you should just play higher stakes, but some people have a differant skill set to others.
Some prefer to play fewer tables well at higher stakes, the problem with that is some people simply aren't smart enough to do that and profit. Also you will encounter and have to learn to deal with a lot more variance due to playing less tables (you will notice being busted out of a tournament), and also the level of competition gets better (especially true in cash and SNG).
Some with differant skill sets prefer to play many tables at lower stakes, which still gives them a decent hourly rate with less variance.
There are cons for playing many tables, as mentioned by pascal, a lot of newer players add far too many tables way too soon. To get really good at multitabling, you need to start out small, playing just a handful of tables, and over literally thousands of games start slowly adding a few tables here and there.
Things like your table setup, hud, and shortcuts will give you a real advantage, also things like using xbox controllers or good mouse/keyboard will help you to play many hours. Also you will need to actually train yourself to play many many hours too if you plan to mass multitable, again, start out slow and build yourself up to it, much like an excersize routine. Adding one last thing, i found that most of these mass multitabling types have kind of a way to play each and every situation, like, they come up with default plays to help them make decisions quicker in game. This comes with playing loads of hours, and reviewing rather than thinking through the most optimal way to play the hand in game.