There are no more scheduled ops on the DC9. At my last recollection they had 1 DC9 left but this too may be gone now as more 717s have been delivered.
The only reason I can think of they would want something other than the MD80/717 is for charter ops...YX
does plenty of sports charters for baseball, basketball, and hockey teams and a longer range a/c would open them up to more business, methinks. But I can see why the cost benefits of mx and fleet commonality might rule against that decision.