I took the earlier poster's message as saying that he felt that Disney "should" do so.Except that Disney doesn't see use of the transportation system as an exclusive perk for people staying at the resorts.
To be fair, though, Disney does hold that resort guests have priority on resort-to-park transportation whenever they wish to enforce it.
In more ways than that which you mentioned. Guests enroute from attraction to attraction aren't occupying the attraction. In that way, transportation also helps reduce needed capacity.It's in Disney's interest to keep as many people moving throughout WDW property as possible.