If the new VS2011 IDE improves the productivity of your staff, that's still a valid reason to upgrade - regardless of new UI/perf in the compiled application. If you have 100 staff and they all become 20% more efficient, you're working as hard as if you had 120 staff - and that 20% spare time that just opened up will make more of a difference to the look and features of your product than any amount of fancy new perf or UI that comes out of the compiler.
Just for instance, intellisense and the static analysis (warnings) in VS2011 use the VS2011 toolchain not the VS2010 toolchain, and hence even if you're compiling in VS2011 with the VS2010 toolchain there's still added benefit.
Whether that's worth the price of the upgrade is not my call - that's up to whoever pays the bills in your company and how more (or less) effective VS2011 would actually make your team.
Now, this is what I would call grasping at straws.
You are right, whether improvements to the IDE are reason enough to upgrade to a new version of Visual Studio is not your call. Let me tell you, I have *never* seen any C++ team to upgrade on that merit alone. Never. The compiler and the tooling were *always* what the upgrades were all about.
Now, this is a big world, so I am sure you will find some developers who wouldn't care much about the compiler and would upgrade just for the IDE. But you'd be hard-pressed to show if this type of behavior is at all typical. We are not exactly newbies here. We do C++ for a living, did it for umpteen years. The compiler and the tooling always go first. End of story.