I've worked on MMOs before in my day, and I've only had a server wipe once, and that wasn't something that affected the accounts of everyone. In theory, though, and based on what happened to the MMO I worked on, this is probably what would happen:
Cons
Server overload, due to many new players logging in because of the spread of news that PokeMMO is reborn
New players complaining (as any admin or moderator of any website knows, every time you update something someone complains, but in the case of something this large, lots of players would disagree)
Possible glitches due to wiping the server wrong (you may think it won't happen, but it does)
Rebuilding the economy
Players will leave
Pros
All players start out equally
New players will join
During server downtime, a chance to implement new features
Rebuilding the economy (both a pro and a con; the economy is terrible to some and amazing to others)
Give players a chance to reexperience the actual game instead of just breeding Pokemon and selling them
I've just been here for two days, so take my list with a grain of salt.