I get what your saying about controlling access, just don't think it would be a big enough benefit from the security point of view to warrant the damage it will do to the community itself.
One of the main attractions Sharecg has for me and many other content providers is that people can download without registering. If registration was required prior to download the first time I dropped by, I probably wouldn't have done it because I had no idea what sharecg was at that point.
I got here from following a link from another website, and it was only to download a small prop file I needed for one scene. Had the website started asking me for all manner of personal information just to download the file odds are good I wouldn't have downloaded it.
It wasn't a super high quality couldn't live without it sort of thing, just something I thought it would be nice not to have to model myself. Since Sharecg was so easy to deal with I stuck around, got a little more familiar with the site, then I signed up and have even started providing content of my own.
But had they wanted me to register prior to downloading odds are good I wouldn't have stuck around, just wouldn't have been worth it to me at the time. I think you'll find a lot of other people will feel the same way when the first stumble across sharecg. Sure, some will stick around and register, but a lot will just come here with one freebie in mind, find out they have to give out their life story and fill out a bunch of forms to get it, bounce back and forth with a verification email, etc.. and run the risk of their email account being spammed back into the stone age for one file - and I think you'll find a lot of potential traffic going elsehwere.
Sure, an automated security system would be nice, I just honestly don't think it's feasible or a good idea given sharecg's stated business model. Again, just my 2 cents worth.