I don't think they should make trainers valid attackable targets. Bad idea. Unless they were the same level as the rest of the mobs in the zone, so we could go to Cap as a 50, take out the Arbiter and impersonate him for a while, but give people the wrong info and refuse to train them.
If each trainer was a bit more unique, or gave out their own missions, we could get them in instanced missions and fight them there though.
@craggy see me on Union for TFs, SFs (please!) or just some good ol fashioned teaming.
Also, have you looked at the SIZE of them, at least the guys? They're as tall as a Council Cor Leonis, so I'd expect them to have quite an ammount of punch behind them.
On topic.... the arbiters have names, and that makes them unique enough to deserve some sort of uniqueness in appearance, therer are of course the basic ones in grandville, so give the important ones importance.
Also you could imput the going rogue mechanics in, if you attack an arbiter then he will ANIHILATE YOU with like one shot or swipe, or you will be destroyed by a group of arachnos =P