Ok. I kinda see why people ship this. Let's think about this.

We have the following:

Fell!Sans x Swap!Sans

Fell!Papyrus x Swap! Papyrus

Swap! Papyrus x Fell! Sans

Fell! Papyrus x Swap!Sans

And, of course, we have fontcent.

So... I guess you really can't argue why people ship this...

(I personally ship Swap x B and Fell x Maestro)