I've been wondering why VS 2005 uses it's *.vshost.exe binary to debug projects... I've seen several problems that it has caused posted in the newsgroups, but I assumed that there were actually benefits that came from it.  Today I came across a blog post outlining those reasons:
http://blogs.msdn.com/dtemp/archive/2004/09/09/215764.aspx

Here's the summary of reasons from the above post:

• Improved performance
• Partial trust debugging
• Design time expression evaluation (using the "immediate window")