The only time I'd consider paying for one of these sites is if there was some genuinely exclusive beta that I *REALLY* wanted, which hasn't happened to date. I haven't got time to play all the full games I want (typically older ones which I know are great games based on reviews etc), never mind countless demos and betas on top.
As mentioned, within a day or two most demos will be on either bygames or the Zen gamershell mirror, both of which should easily max out any home internet connection. They are also both direct download links without any need for 'logging in', using proprietary download managers or any of that nonsense. Just your standard FTP client, HTTP download manager or even web browser will do.
In fact, thinking about it in the 8 years I've been online, I think the only internet-related thing I've ever subscribed to are ISPs.