I'd say, it's not a good choice. Let me reason out why... 1. Most of all the GC servers are currently running on 1.8.x/1.9 already, we don't want a "3rd version" of another server. 2. Also, in this way that hackers can be able to escape Admins even more easily when using 1.7 hacks and 1.7 clients. 3. Most of all, the spawn's blocks and waiting in-game lobbies might not work as good as today's. Certain blocks are incompatible with 1.7.
I disagree with the 2nd point because the version doesn't really affect who uses clients and there are many 1.8 ghost clients too if thats what your talking about. The other two points I agree with but I would still want practicepvp to be 1.7 compatible for better pvp experience.
