Essentially I say what
Andy_P said.
I would add that you have to look at it in the context of Windows 7, in that when Windows 7 was introduced, Windows XP was still supported.... so the XP feature was introduced to ease the migration pain. However, now Windows 8 is the current version of the system people should have by now really been fully migrated over to Windows 7 and no longer be relying on the XP VM.
So yes, in essence, you should not be using Windows 7 XP Mode just as you should not be using Windows XP. In particular I would highlight the following bit from Andy's post ...
and any vulnerabilities should remain in the 'sandboxed' guest OS..
"Should" being the operative word.... there are exploits out there that can figure out they are in a virtualised environment and actively seek to work around that temporary obstacle in their path.
In the end we're talking about software virtualisation here, so the segregation is only as good as the memory management (and other aspects).