In the case of IMDb, I sometimes get the "take a rest" message when using the site manually, but I'm not aware of them having blocked or restricted the plugin.
I never got such "take a rest", but after using the IMDB plugin to get episode info on 8 seasons of a series without problems, the next few seasons always skipped one or two episodes with an error "page not found". After waiting a few minutes, i got the next few seasons again without problems. The same happened again at some other time of day on another series.
btw: the errorbox for "page not found" had only one button for confirmation. would it be possible to add a second button for the option to retry ?
Since the number of pages multiplies if someone also gets people info for each episode's actors, i thought it a possibly good idea to restrict oneself a bit before they implement stricter limitations, and not flood a site with requests when not necessary. Since we don't want to mirror the entire site, a little slowdown shouldn't hurt us too much.
The method for the slowdown (wait after each movie, wait after each page, wait after each file, wait some time after a burst of one minute, etc) remains to be discussed later if such kind of feature would be in the planning state.