Reasons to buy him out:
1) You want to clear playing time for younger players.
2) He's been a good soldier, and you want to let him find a spot on a contending team. Down the road this improves your reputation with other players.
Reasons to not buy him out:
1) You want the opportunity to resign him next year, especially if you trade Bass over the summer.
2) It's possible you could obtain some future value for him in a sign-and-trade.
3) You want your team to continue to play its hardest to create good habits for future years, and cutting Hump would not be consistent with that position.
Reasons that don't matter:
1) Money -- C's ownership could save a trivial amount of money buying him out (by their standards) -- at most a couple hundred thousand.
FYI -- Hump will get paid what the Celtics owe him. Negotiations in a buyout are regarding how much of any contract Hump signs for the rest of the season count against what the Celtics owe him. Per the CBA, the Celtics could release him today, and owe him the rest of his contract. If he signs elsewhere, for say $1 million, half of that new contract would count against what the C's owe, and so their obligation to Hump would be reduced by $500k. Hump comes out okay in this scenario, because he makes $500k more than his original contract with the C's called for. That 50% is the default set by the CBA. If Hump asks for his release, the C's could negotiate, and try to have more of his new contract "set-off" his original deal. Perhaps they agree that 75% of his new contract counts against what the C's owe. Now the C's would owe $750k less to Hump, but Hump would still make $250k more than he would if he didn't sign elsewhere. So it's not a money decision.
2) Roster spots -- we already have one open. The D-league only has so much to offer.