It's quite useful for most of the non-combat skills, but definatly not a good idea to rely on them for things like potting to tb/barrage. Would really suck if it ended up lowering magic by 6 instead of raising it.
Yes, they're scammers. While you're fighting they will "accidently" drop 4 of some random noted item which protects over a whip, usually mystic tops. Then they kill you.