I've run games with cohorts allowed, and played in games with cohorts, and never seen a problem with it. It often lends quite a bit of extra colour to the game too.
One thing that I've seen done that worked quite nicely was to have the cohort as the 'backup character', so that if the main PC dies, the player has (a) got someone to continue playing with in the adventure, (b) has a ready made party member that can be 'bumped' to the appropriate level (often they are two levels lower than the PC, and a replacement would be one level lower) and fit right it - after all, they have already been adventuring with the party for yonks.