Like some other people, I have slow and erratic internet, and on a bad day downloading a 600MB file can be a trial. I finally managed a complete download of the new Salix Live RC on the fourth attempt, after the others died half way - one at 450MB. But md5sum indicates it's a bad download, so I have had to start again this morning.
The great thing about torrents is partial or faulty downloads can be corrected by redownloading only the affected parts, so you're bound to get there in the end, however slow or intermittent your connection.
I imagine the policy is not to put up torrents for beta isos because there isn't the demand to make it necessary. But there are other advantages, namely ease of download.