Not allowin root login with remote services like ssh also adds to the sercurity because you have to know two passwords instead of one to get to a root shell.
Don't know if this is like this in other *nixs too, but in openbsd, if the weel group is populated, only members of wheel can su to root. So if you make sure all members of wheel have strong passwords and so does root, brute forcing youre way to root would be considerably longer...

Ammo