There are enough merit sources and merits in circulation for a fair distribution of merit. More merits in circulation would mean more merits in the hands of merit abusers which isn't something we would want to see happening around, do we?
It is Theymos that adds new merit sources, and to become a merit source you have to apply for it, and there are requirements to apply for the role of a merit source in Bitcointalk, one of which is the user must be a reputable users. Theymos is not going to make users who are not reputable MS's, and the chances of reputable users abusing their source merits is very slim, so there is nothing to be worried about.