I would strongly advise against it for the sake of the entire Crypto space.
Absolutely agree- although I'm tempted to think in this case we are faced with either a definitely dead project or a questionably dead project. We could fork it now because practically no one is using it. After all this time we only even have a Beta 0.0.8.2 that compiles only on linux. There are altcoins with joke names that are more prodigious.
Additionally- forking Mastercoin is very different then forking bitcoin. Mastercoin doesn't require anyone running a client in order for it to 'exist'. It's there in the blockchain. If only one person is happy to stay on the old Mastercoin their coins are just as secure as they always were. So it's not that we are forcibly withdrawing the hashpower of one blockchain to another, thus destroying the previous chain. We're simply choosing to recognize different tokens. Although the argument is worthwhile discussion. It really is not a nice precedent, but perhaps necessary.