Last call draft means exactly that. It's still a draft, so if you see issues, let them know. But this is the last draft, so speak now or forever hold your peace on changes to GPLv3.

Richard Stallman has also released an essay on the draft, providing some reasons he sees to upgrade, but also including this word about compatibility:

First of all, it is important to note that upgrading is a choice. GPL version 2 will remain a valid license, and no disaster will happen if some programs remain under GPLv2 while others advance to GPLv3. These two licenses are incompatible, but that isn't a serious problem.

When we say that GPLv2 and GPLv3 are incompatible, it means there is no legal way to combine code under GPLv2 with code under GPLv3 in a single program....

Fortunately, license incompatibility only matters when you want to link, merge or combine code from two different programs into a single program. There is no problem in having GPLv3-covered and GPLv2-covered programs side by side in an operating system. For instance, the TeX license and the Apache license are incompatible with GPLv2, but that doesn't stop us from running TeX and Apache in the same system with Linux, Bash and GCC. This is because they are all separate programs. Likewise, if Bash and GCC move to GPLv3, while Linux remains under GPLv2, there is no conflict.

So, no disaster if you wish to stick to v2; but there are some legal benefits to upgrading.
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