many of those recycled actors played characters with little or no prosthetics. This was going on since the original series and plenty of other shows before that, as well as shows today. (The various "Law & Order" shows do this a LOT! The "CSI" shows do it too. So did "NYPD Blue".)
The Law & Order
universe (which is only SVU now) makes gleeful use of actor recycling because it frequently raids the casts of any and all Broadway shows. (Go to any stage show in New York City and try to find the actors in it who haven't
appeared on L&O at least once. You won't come up with a long list.)
And it's a win-win, because the actors can do L&O in the afternoon and their stage shows at night, plus L&O gets better actors as a result.