A thought about all of these premium stand-alone buildings

The incentive (if you can call it that) is that fewer and fewer buildings that pay out are given away for free or earned on a prize track anymore (just look at this event's free building to donut building ratio). The further along in the game you get the more lopsided it becomes. Eventually, if you wish to continue the town-building aspect of the game in earnest, premium buildings will become your primary avenue of addition. So really, no incentive is necessary because sooner or later it will be all there is.