I think multi class characters receive too much bonus with relatively little penalty; falling 1, or at most 2 levels behind single classed character. (in most instances only 1 level behind. the same amount of exp will make either level 7 fighter or level 6 fighter/theif, the latter being vastly superior, of course.)
Shouldn't the experience progression curve be adjusted so that they fall at least 3 levels behind single-class characters? I'm not suggesting to impose a penalty, but rather change the level progression rate to allow single class to advance faster.
As of now there is absolutely no merit to play a single classed character as a protagonist.