I'm mainly using Google Play for my movie collection and I'm generally only buying when they're on offer. Probably not paid more than £5 for any of them and they're generally only movies I've actually seen before and I know I would watch again as I really like the film. Kind of odd I guess, I wouldn't buy a movie I hadn't seen unless I was really sure I would like it... and it was under £5.
I have rented a couple of times, again, when it's been a Google offer of some kind, like for 99p. I guess it's a psychological thing I don't want to pay for something that is a one use thing.
Yes, there's a risk they disappear, but as I say, if I've only paid £3 for it, I'm not all that bothered. I prefer that risk but having my movie collection with me everywhere I go.