Most decoder manufacturers sell their decoders with different harnesses; one is for the NMRA 8-pin, one has no plugs at all; these are the ones that are hardwired. Of course, some are made to fit a specific loco (like a Kato).
I had a handfull of no-plug ones: I sent a few to my pusher who put a NMRA 8-pin on them, so now I have an assortment of harnesses to fit whatever situation I run into.
And, this is important to me, because when I grab a loco to do an install, the last thing I want is to not have a part or item to complete the task. When that happens, usually I lose interest in completing it, and it adds to the locomotive dead line. As in, locos disassembled, parts lost, no decoder, lost track of what needs to be done, etc., etc. Waste of time and money.
Kennedy