First off, buying plenum just because you like working with it is pretty insane... Plenum is actually used because it doesn't produce toxic fumes when it burns.

...

Wire is cheap, time is expensive.

But I've never cut a wire when using plenum wiring, and have never had to redo a run or connection because of it. Given that time is expensive, that tends to, IMHO, offset the additional cost of the wire.

And I don't know that that hard insulation is necessarily specific to plenum wiring. It probably is, though. Teflon, I think.