I attempted something with players leaving the party once, but it froze the game for a few seconds before I could actually switch through different party members. But I don't see the point in having a stationary ally just randomly in a game unless it's for reaching a place far out of your range, which is where the ally toss feature makes it have more sense.
Also, I only said I liked the motion trails. That's not saying I like the X.A.S. system for it, you could just as easily duplicate the motion trails to any system.