As discussed in bug 161953, Portage does the right thing to resolve a virtual to an already installed package, which causes users problems in case of virtual/libstdc++ as you can read in this¹ sticky thread - and there are a lot more of these, discussing the very same issue. You might suspect that some newbies even manage to break their toolchain or system by installing incompatible GCC versions, filing invalid bugs, here. So, given that we do not support older versions, masking them would solve the problem, without holding users back to install older GCC versions, if they really like to. [1] http://forums.gentoo.org/viewtopic-t-577369.html
Good luck w/ this; also see Bug 201035 *** This bug has been marked as a duplicate of bug 183766 ***
