My understanding is that Reaper has no distinction between mono and stereo…I guess if given a mono source it is doing exactly the same thing as being suggested here.
By forcing you to this behaviour I’d argue things are less confusing for newbies. They only need select mono/stereo in one place not both as input and channel as we are used to doing in Cubase.
As to mono effects on mono signals only…well I’m not convinced that a stereo compressor on a mono file/stereo track will produce any different effect than a mono compressor on mono track. Even if the plug has unlinkable channels it will apply the same effect because it has the same input. Maybe this is worth a null test for confirmation at some point.
So for me the last reservation for not using stereo tracks fed from mono is I think the CPU use. I may run a test project and see just what that difference is but it seems to make sense that 2 channel of effect will use more than 1.