First of all, hybrid characters (particularly those of mixed roles) are not supposed to fulfill a role fully like a single classed character. A fighter-wizard will not be able to defend as well as a fighter for as long as a fighter because it is not designed to do so. He can defend, but may need to pull out from defending sooner than a single classed defender.
Hybrids are supposed to give you some flexibility. If you happen to need to be a controller more than a defender the fighter-wizard will be better than the single classed fighter in that instance. This is the advantage hybrids give you.
Second, different hybrid combos vary in power level. Some combos will be very good, others not so good. I think that you need to ensure that the best combos are not too powerful rather than bring the power of the worse combos up. You not only have to do this for existing classes, but also for any future classes. Making exceptions on a case by case basis is impractical.
My experience with the two hybrid characters in my gaming group seem to indicate that the flexibility can be very useful. The flow of battle can change and that extra flexibility can be quite useful at times. Also, the two characters I have seen seem to have a different level of effectiveness. Still even the less effective hybrid character is still effective enough of a character. Sure it's not highly optimised, but I don't see that as an issue with my group of mostly non-optimisers.
One thing I find about the value of hybrids and multiclassing is the coolness factor. I think it is cool to have a character that is a mix of two classes. It may not be as effective as a single classed character, but sometimes players want this coolness factor at the expense of optimisation. This is the case for single classed characters as well. As long as the character is at an adequate power level even if it is not near the best, it is OK.
As for the effectiveness of the most optimised hybrids compared to the most optimised single classed characters, I do feel that optimised hybrids for most hybrid combinations are not as effective as most optimised single classed characters. There are a few that are very good. I think from an optimisation point of view, you could shift the power of hybrids higher or lower. It seems that how high or low you set the power of hybrids is the issue. From my personal experience with my particular group, I feel the power level works where it currently is.
With hybrids, I feel that they have to give up some class features that single classed characters have. I personally like the fact that Hybrid Talent cannot be taken more than once (except with the paragon tier hybrid option). One of the reasons is this coolness factor. For instance, single classed characters get a number of class features that are not available or not available fully to hybrids due to the limited built in class features and hybrid talent. For instance, hybrid wizards cannot gain the spellbook feature. This is a cool thing that is only availble to single classed wizards. If you want this feature because you like it, then you have to choose to play a single classed wizard and not a hybrid wizard. I do agree that being able to take hybrid talent to gain more class features may not be broken all the time. Some hybrid talent options aren't that good. However, I do think that not allowing HT to be taken multiple times give single classed characters some coolness factor that is not available to hybrid characters.