Did you do the check on open or on creation of the symlink?
We explored both ways -- and rejected them.
If you are doing check on open -- it must carry a price tag. On a shared hosting server, open is pretty common operation, and you just made it costly.
Also, to properly defend against race conditions it has to be atomic, which makes it even costlier.
We really didn't want to make performance worse, if we didn't have to.
If you did it on creation of symlink (we actually know about hosting company who did it that way) -- it wouldn't work for us as well, as it breaks some installation scripts, like WHMSonic ShoutCast Admin Pro (the script makes a link as a user, for each user, to sc_serv binary that is owned by root). I was pretty sure there other apps like that - and I didn't want to deal with all the support questions.