POF doesn't make it a requirement, probably because it would create friction in the signup process. eHarmony on the other hand, that's too many questions and no one is going to fill that damn thing out.
Let people sign up without filling in much to just browse. But at each and every step, show them some possible candidates that will be unavailable to them unless they fill in a bit more information.
"This pretty woman? She won't let you message her unless you fill in your birth date and it matches her preferred age range. Guy over here? Cares deeply about your music taste".
It's no different from how a lot of these services will suddenly hit you with paywalls to do various things. Get you fired up about contacting someone first, and then demand just a bit more.
Out of all the online dating sites I've seen/used I think they are the best by far.