Actually, I found the opposite in my games. Wizards really shine. The reason, I think, is the change in setting assumptions.
For example, if you want to have a village of 100 people, they're ALL 1st level commoners, maybe 2 first level warriors, maybe an expert.
The fact that if they all stood in a crowd, the wizard could kill them all with a few words (I'm talking fireball here) - that's scary. A fighter knows he could survive that, but who knows what else the wizard can do? Turn invisible? Protect himself from arrows?
Basically, if you cap at 6th, there's less of the arms race: "but my spell beats that" - "but my goggles counteract your invisibility" - "but I've got boots that do that anyway" - "WE'll just raise him from the dead"
Magic is powerful under these rules.