If you intend making a living from on-line Poker I think it is a must to play multiple tables, but it does require a lot of concentration and be aware that miss-clicks are a common event, if you do not set up the table layout properly.
I used to multi-table years ago, but found it took the enjoyment out of playing, I like to study the other players at a table and try to identify when a player is raising with a decent hand or just trying to steal blinds, it is hard to do when changing from game to game as you cannot keep track of all the action on each game.
I play Poker now just for the enjoyment of the game and even when I get sucked out on, I do not let it get to me because I know that sometimes Fish do suck out, It's the nature of the game, If you are holding the best hand before the flop, it does not mean you will be holding the best hand on the River, You just have to go with your gut and take the consequence of the suck out. It happens, That's Poker!
When the fun Stops, STOP!