They (5th teams) receive (I guess) better attendance than play down teams next season (fan survey should reflect that) and no worries about demotion (sometimes that's a lot). of course you can even argument that 6th (and income from 3 matches - 2/3+1/3+2/3 of gate receipts) is better that hopeless 4th in play offs against a dominant team (and income from only one match - 1/3 of it). After all it all depends on situation in the league.
Certainly I would support "extra regular season matches" between 5th team at date of conference semi finals (home court advantage to better of the two or random) and conference finals. Suggestion forum...