Also, upgrading your C library is dangerous. It's just about the only thing in the system where if you mess up installing it, or build a broken version and don't run 'make check', you break your whole system at once with no way back without use of an emergency boot disk, a netboot, or a statically linked emergency busybox shell (which you must have prepared in advance, of course, as with no working libc you're not going to be able to compile or install it after the fact). None of that is true of the kernel, unless you're unlucky enough to install a kernel that scrags your filesystem.
I'm happy to install new kernels frequently. I'm much less happy to install new glibcs frequently: I'm on tenterhooks even when a distribution glibc upgrade comes down the pipe, because I have known *those* to scrag my whole system in the past as well.