I'm totally fine with it being optional I suppose, but as a player I don't really see the point of doing an RP without it?
If I don't want interaction I'll just write my own story, without anyone else getting involved whatsoever. If I have my own character arcs, interactions with, in this case, NPCs and a story to tell that doesn't involve player to player interaction? Then why would I ever bother doing RP in the first place? Obviously it's different for everybody, but from my perspective at least I would have no reason to RP in the first place without the interaction aspect.
Getting to write a story where everybody plays a different character and those characters talk, fight and develop together all the while bouncing off one another for their own personal development along the way. That's what RP is to me, in essence, I can't grasp the appeal in it otherwise.
Like Alissal however, I have also experienced a variety of other Roleplay mediums outside of forum based so my view might be a little biased in this regard... then again I did have my start in Roleplay right here on this very forum, so I'll stand by my viewpoint.