Yes, mono plugins are really convenient because they do connect the pins automatically to mono, but unfortunately mono plugins are very rare. I don't use Waves or Duende plugins. Those are some of the most expensive plugins around and not really worth that kind of money in my humblest opinion... but YMMV, of course. I do use some Fabfilter plugins and they have separate mono/stereo versions of plugins. d=
Some rare plugins I use have a mono switch [Sonimus Satson and Britson] and that makes the job much easier, but 90% of plugins I use neither have a mono version of the plugin, nor a mono switch. So if Reaper had a mono audio track, that would make it much easier for us who record a lot of mono tracks and like them to stay mono all the way to the pan pot.
And as I said: mono processing cuts CPU consumption in half with many plugins, and that's always very welcome, eh?
Some stereo plugins use double CPU processing even when you set them to mono mode in Reaper, because they "see" dual mono input. I've noticed that when I use same plugins on a mono track in Cubase that they use almost 50% less CPU. It seems like some plugins need to be forced into mono mode somehow differently than Reaper does it currently when you unpin 2nd channel on the input and pin both 1st channels on the output.
Cheers!