The mdl file format is only as obsolete as the company who programmed it. It is a generic format which varies from engine to engine and is not specific to anyone. If you are thinking of the old style mdl (Quake I believe) than yes, that is pretty obsolete. The A5 mdl is not the same. Also with the upcoming bones, the model format will still probably be mdl. Why, cause it looks cool

Other engines use there own mdl format (Neverwinter Nights for example), don't let the extension fool you though, they are not all the same.

As for add-on plugins for md3 or Quake 3 bsp files...it probably could be done, but why? Not to mention that their would most likely be copyright issues involved with something like that.