The reasoning behind not using mono as backing is that mono stretches
Supposedly, if and when you have a great deal of mono backing out, it will reel in "stretched". Afterwards when it contracts back to normal it will crush the spool, or at least cut into the remaining backing underneath and tangle.
I've considered this often and it seems pretty far fetched to me.
Not that I'm going to be the one to try it and prove otherwise ....