No: the compatibility modes aren't the same as the real browser - they try but some features, especially things like CSS or DOM handling of errors, non-standard properties, etc. will not be consistent enough to avoid testing in the real thing.
Compatibility mode isn't what he's talking about. In the developer tools, there's also "browser mode", which allows you to select a different user agent, and "document mode", which allows you to select a rendering engine from IE7 onwards.
I've been using these for the past few months and they've worked fine. Plenty of things I wish worked better, but for the most part I'm happy that it's free and I don't have to reboot just to debug some CSS.
VMWare Fusion's importer works perfectly - in the past there used to be a concern with drivers but at least by VMware 4 it just works.
2. Use unrar or http://wakaba.c3.cx/s/apps/unarchiver.html to unpack the segments
3. Open VMWare Fusion & choose import
4. Get coffee & wait for disk I/O
5. Install VMWare tools & pending Windows Updates
6. Take a snapshot so you can reset it after the timelimit sets in (as noted by Microsoft, they disallow storing data durably for more than 90 days)
There are utilities to convert a VirtualBox VM to VMware, if you are feeling adventurous :-). From my experience though, it's kinda painful (I forget what the issues were but I remember regretting the decision to convert the VM).
Presumably it's useful for web developers on Windows 7 or above, but who need to test on IE6/7.
As to why they'd use their own VM in preference to VirtualBox I don't know. But to be fair, Microsoft isn't the only company that neglects developers who aren't on their platform; "not invented here" syndrome seems to impact all software companies eventually!