This is from Rapidshare:
Believe it or not, I've been trying to download a file, and have failed for 5 consecutive times because of these insane CAPTCHAs!
I think it was very much intended to be like that, to lower the stress on their servers. If you're downloading a file of say, 100MB, their system lets you wait 3 minutes (through a counter) before letting you download it. Since I've failed for 5 times, this is 15 minutes more relief to their servers. Multiply that by thousands or hundreds of thousands of requests (rapidshare.com ranked #12 in Alexa) and it would make a huge difference to their servers.