We all do it, but no one really has taken on the task of analyzing the concept, that I've read anyway.
Why I do it
I multi-table $1/$2 for bonus purposes, only. On most sites, to clear a bonus requires a minimum raked hand. The size of the rake does not increase the rate of bonus clearing. Therefore, in my opinion (although frankly this should be a given since who's opinion can I give, other than mine), it makes little sense to risk more money for the same return. If you're playing 4 tables of $3/$6, how much difference will a bonus make?
Why I don't
Playing more than one table decreases the attention you pay to each individual table. Now, the bad boys play a ridiculous number of tables. How much did their setup cost them, to buy 2 monitors capable of super-high resolution and the horsepower to run them? How long did it take playing multiple tables to pay for that equipment? How much did they lose while playing multiple tables just to buy high-end equipment to play multiple tables? Where's the +-0 EV in any of this? (What am I missing?)
Leaks are your worst enemy. They are pernicious--which means they'll constantly sneak up on you, grow and cause you to lose your money. You have to guard against them, and know they'll find ways to make you waste good money. Playing multiple tables is a great way to develop leaks, and an easy way to not see them. Beware, or accept that you're willing to give up a little for whatever gain you'll get by playing several tables at a time.
Why I would
It does spread the risk, I think. When I play 4 tables, I can't think of a time I was up on all four. I'm never sure if it's me, the cards, the table or the gods. I always wonder if I was just playing that table, if I'd still be losing, and not knowing I'd be winning on another, and winning big. I'm not sure there is an answer for this question. Perhaps it's really the poker gods.
Playing more than one table keeps me from being bored. This is a really dumb reason. Also, I believe, a dangerous reason. Playing bored is wrong. If you're bored you're not paying attention. If you're not paying attention, you're losing, even if you're up for the moment. Maybe this should be under Why I shouldn't?
No doubt some math geek on 2+2 has the mathmatics to prove that playing multiple tables is +EV. For him, at least, he's proven to himself it's profitable. I'm skeptical, if not of him, then certainly of myself. I'd sure like to know.
CJ