On a single table you can get an excellent feel for all your opponents, detect tilt much rapidly and are aware of the texture of the whole table at any particular instant. Because of this I play much better single tabling than multitabling and so do most of the people who customarily multitable.

In general most of the people who are multitabling are playing ABC poker which basically is good enough to make a reasonably consistent profit.

It is not uncommon to be heads up on two tables simultanously with the same player. Or to be on the receiving end of a possible blind steal by a player while you are doing a steal on that player at another table.