I guess the problem is that it stops working when you start being big enough to attract the spammers' attention and get bots tailored to your registration syste

I guess the problem is that it stops working when you start being big enough to attract the spammers' attention and get bots tailored to your registration system. In the meantime, I guess this is just to avoid problems with general stupid bots which just fill out all the forms they can't find without trying anything fancy.