Ofcourse you don't want it inside of your bases, however you also have to consider that lava can be used to build traps. "You activate a lever and lava comes out of the wall"
Which would be one of the "cooler" traps, in my opinion.
Restoring chuncks regularly is not a good idea, I remember Cryston telling me that it takes alot of the server to restore a certain area. Now, I don't know how much this applies to the PvP server ( in terms of hardware etc) but I'm pretty sure that if you reset a 100x100 area every day it would be very difficult. The drain command is alot easier to use. Now that would however take time off of admins, since they would have to fly around and look for these "Lavagriefed" places.
You're idea is not bad tprez, I just think that if you make a pe saying "this area contains alot of lava, the responsible staff will go there and clean it up, takes about 5 seconds. That would however, mean that players have to help to keep the server "clean", which not all people are willing to do unfotionately.
All in all, I think lava should be able to flow on all blocks. Maybe giving people the option to have it on in their repsective region would be a good thing.
Well, my thoughts anyways <.<