Whats wrong with 32bit apps on x64?

I recently upgraded to Vista x64.* There are a few apps that have 64 bit equivalents, but most of what I've installed has been 32bit apps.* My question is why bother writing for 64 bit?*

This question obviously doesn't apply to an app like SQL Server where it would use more than 4GB of ram.* I'm talking more for stuff like Firefox. a project that's trying to port Firefox to x64.* I'm curious as to what's the benefit?* The 32bit version seems to be running just fine on Vista 64.*

Is there some performance benefit you should see with 64 bit apps on x64 over 32 bit apps on x64?