Obviously, if you've a "small-ish" apartment/house and getting good coverage with a single wireless router then why bother?
If you've reasonable coverage save for one remote corner of the house you might go with an AP or range extender.
If you've multiple weak spots then mesh might be for you!
I don't know for sure but, from what I'm reading, it looks like most mesh systems include the "router". If you were to keep your main router that might complicate the "one button" simplicity mesh providers brag about.
Asus is trying to get people to keep their Asus router with "aimesh". Some like it, others say it ain't ready for prime time.
Myself, I considered mesh briefly when I was upgrading my from my N66U but, at the last minute, I went a Blue Cave (on special). It's cute. Cute enough that my wife let me take it out of the corner and up off of the floor. I now get "whole house" coverage : -)
I think if you're dealing with at least 2,000+ square feet, that may be time to start looking at a mesh system. Under that footage I'd wager that 1-2 routers could handle whatever you need, and the setup to make that work wouldn't be too arduous.