[Anoree]

Some sites offer testing them for a couple of days, then switching to a monthly fee or something like that.
If these sites had no download limit, somebody with a big broadband connection could use the test time, suck off anything he can with download managers or site mirroring software, and sign off again. This way you could get high amounts of content while paying next to nothing.
Meanwhile the normal users might experience reduced download rates as a side effect.

With FTVGirls it'd be a bit complicated: How many MB / GB per day should be considered normal behaviour, given that the video files get bigger and bigger? What is the reaction of members who see "Download limit reached"? I think Rob sees a burst in downloads when a new update is set up. How should that be handled?

The members' satisfaction and return-rate has to be higher without a download limit in Rob's case. This probably outweights the loss seen when members grab off the content and don't return.