I'm exclusively using Parallels to check out web pages we develop in IE6 and IE7... not sure I'll bother with the upgrade right now...
That's a great use for it and I know several other web developers who use Parallels for that very function as well. Testing compatibility for webpages with IE is obviously crucial and having to use a second dev box or having to reboot into Windows via Boot Camp is a hassle in this type of a situation.
But yes, in your case I don't know if you need the upgrade to version 3.0. The question is, you may not need the upgrade, but do you want it?