Well, a Bitcoin-mining MMO game probably should be free. It would have a larger install-base, and people would be more tolerant of a miner running in the background.
Also, MMO games generally have pretty low graphics requirements (to appeal to the widest possible install-base), so a lot of gamers have excess GPU that could be used for mining.
I think miners also need to download a lot of data initially (i think 500GB). Mining might require network access which has a negative impact on network latency, and a negative impact on most real time MMOs.
i.e. there are also a whole pile reasons why running a miner would pissed off your paying customers.