Ok, I see your point (about the spam) because it could lower the barrier for bots possibly. However, our community is tightly knit (members can join only by invitation) and trust is high, so I don’t anticipate this being an issue for us.
With that said, maybe this feature could be configurable so that admins could turn it on if they want it and leave it off if they don’t?
In thinking how this might be implemented, I’m envisioning a format like this:
pm+USERNAME@myforum.site