There are lots and lots of minecraft servers that run without checking if an account is registered with mojang, and these do not get any more problems with multi-accounting than others. You do not have to worry about that: servers could use capthas, or have “new players per day” limit et cetera.
Also, i am not sure how sane the idea is, but is not it a good to dedicate more computational resources to players who play longer? So if a server has 10 players, the oldest player gets a maximum of 19% of computational time, but a player that just joined only 1%? The actual method of allocating would differ, of course, but how good or bad is the idea itself?