The ASP.NET 2.0 compilation model no longer requires you to pre-compile your .vb files - they are compiled on the fly. Shared code is typically placed in the App_Code folder, while code-behind files reside in the same folder the .ascx is in. The bin folder is technically speaking no longer necessary in ASP.NET 2.0, but it's still supported for legacy purposes.

The only substantial performance enhancement pre-compilation provides in ASP.NET 2.0 is with the initial compile, so unless you aggressively recycle your application pool and have a substantial amount of code that must be compiled, you won't see much of a difference at all. The primary benefit of pre-compilation is for developers who wish to distribute their applications without the source code.

Clearly the bin folder is still supported for backward compatibility, or where for example you have to use legacy .dll's in your application, as I mentioned before. However, a pure ASP.NET 2.0 app that doesn't rely on any third party applications will have little use for this folder.

Bingo - that would be your performance gain.  In a number of posts related to performance, one common question has been if you're running source vs. install.  The only different scenario would be under a 64 bit IIS install.  I'd like to see the JIT compilation performance there on DNN. 

You're right - unless you've precompiled the code he will need to keep those files.  I scanned my 4.3.1 install version and did not see that many.