Casual was made to appeal to newer players, who just want to click play and be put into a server rather than have to learn how to join a server. Also if you want to play in a community server, there is the teamwork.tf community quickplay option to find a server, as well as the usual server browser.
Personally I don't mind casual, and it's relatively easy to queue with your friends, and you can now queue for games that your friends are in, which is nice. It also means that no one has to be on a server that is virtually empty for an extended period of time, as (at least in my experience) the game often places you in a server that (once everyone has joined) is at least half full, and generally around the 18-24 players mark.
It is semi-full untill the matchmaker gets an aneurysm and forgets how to fill empty slots. Quickplay didnt have such issues. If you joined a semi-full server it would usually remain full throughout your playsession.
With casual quite often matchmaker forgets how to do its job and you are left with 2-4 player teams for multiple rounds untill it regains its sanity.
Quickplay was superior to casual, period.