I have worked for several years at a company that streamed movies over the Internet for rent. I know a bit about the industry and what limits there are in the technology. The movie industry is very conservative, keeps variety down and prices up which suppresses competition between service providers, while the service providers have ways to lock in the consumer in subscription programs. As far as I know, practically no online movie rental business is making any real money on rental, which means that the risk of your service being cancelled is quite possible.
Streaming a movie to you over the Internet does cost money, every time, so the provider would lose money on you if you watch it more than what they consider to be average. I would not be surprised if a provider would define "buy" to actually be a form of rental, such as "watch up to n times" and/or "watch within n months".
I think that it will take some time before movie rental/buying online becomes a good choice for the consumer, if it ever does. For the time being, I'd rather go to the place on the corner and rent a physical DVD.
I would absolutely never "buy" the rights to only stream a movie. In a hypothetical scenario, maybe I would stream a movie if that was included in the online purchase of a physical disc, so that I could start watching a movie while the disc is still in the mail -- but that would be it.
The biggest reasons against buying the right to stream have already been mentioned, but I could mention a few more:
* When you stream it over the Internet, the quality will usually be lower than on a DVD. On some devices/systems the quality could vary with available bandwidth over time. Some services offer "HD" streaming, but the bitrate (which translates to quality) is usually relatively low anyway, and you will need a fast, stable connection.
With a physical disc you will always get the best quality available.
* With a BluRay/DVD, you will get extra material, commentary tracks, etc. that you won't get with online streaming.