Hmmm, I just tested this on my site on a non-Admin regular Registered Users user and it worked just fine with the Display Page set to Edit Profile (is yours set to that?) and Yes, Always. If an admin is missing the field, then it doesn't force them to fill it out. (It's been a long time since I played with that setting and I thought it left old users alone, but apparently it shouldn't.)
So, if you tried it on a regular user and it's not working, I'd suggest disabling all your modifications.