There are many good ideas for fixing server congestion, but as far as i remember, the login restriction was in place for the Duty finder server, not the regular ones. My server at least, was fine as far as everyone being on. it was the sheer amount of people doing instances that caused it to stop working, which led to the restrictions to my understanding.

A buddy pass/referral system would be nice though. even more so if said system showed you which server the friend who sent you the pass was on. (save the trouble of your friends forgetting which server you are on ect lol )