IMO data caps hurt online game sales(like Steam games). I have a 4G wireless internet and with that I cannot really buy and download any of the big titles, since they tend to be gigantic in size. Downloading just one of them will mean that I have used up over half my monthly limit. Also, I must be very careful about using Youtube. I have to check if the video is at 360p or not, since going over that would cost valuable data limit. That only lessens my problems. Sometimes I would like to have some sort of 2h play while I do smth else, but that would take a sizable chunk out of my data limit. Playing online itself is not that worrying, since I would probably spend more data limit out of game than while playing. And my data cap has pretty much made me not play those games that make it mandatory to save saves on the cloud.
Like someone else said here, having a data cap on a high speed internet is like having a Ferrari that has just enough fuel to drive 100 meters. I would settle with slower speeds in exchange for bigger data cap, since only time I use the entirety of 4G is when I download stuff, but this internet is not meant for downloading stuff, so yea.