The Master Unit List shows availability for some Primitive units all the way up to the end of the Star League era, before they drop off a cliff. For example, the Terran Hegemony list has the KY2-D-01 Kyudo, the RFL-1N Rifleman, the SHD-1R Shadow Hawk, the DV-1S Dervish, the GRF-1A Griffin and EMP-1A Emperor.
If some Primitives hang on as late as 2780, I don't see why a handful of them might be operating in areas where better 'Mechs are hard to come by. The biggest con, of course, is a continual supply of spare parts.