Would you buy it if it was mined by millions of people? I know I would.

If EVERYBODY started mining it, it would then be used by millions as currency
You cant have the world mining and selling their coin and not have a % of the people that would use it as a currency as well. This would bring value to the coin. The first coin that reaches mass adoption wont be harmed by widespread use, as that is all it needs to be profitable and successful.