How many C++11 compilers are there? This support matrix seems to indicate there are 2 compilers in the lead, with 5 providing fairly decent coverage. How popular is XLC++?
And anyways, adding another compiler is something you choose to do, and usually run a bit of tests for, right? Unlike networking protocols where it's invisible to you that someone has some idiotic inspection hardware that segfaults on certain packets and drops your connection.
Edit: Also, even if two compilers are feature complete, it doesn't mean they implement the features in perfectly compatible ways, right?