OK, so I just swiped this impressive-looking weapon from the T'Rang. Good damage, good effects, casts Nuclear Blast ("Mindblast" rod?). Only usable by non-Faerie Monks, Bishops, and Psionics, but that'll work for the moment.

But it's a polearm? Hello? None of the classes that can equip it have that skill! So they can use it--at a fixed, non-improving skill value of 20.

Is this an "oops"? Why isn't this thing a Staff?

I don't think it is actually an "oops". Both the stun and shock rods that the T'rang use (and which Sadok sells) are also polearms, though usable by a wider set of classes. The better of the two (stun?) is a really good early weapon for Vi or any other polearm user if you reach Sadok early.

I look at it more as a "curiosity" -- one of those things you'd really have to contort yourself to make use of (a bit like building a faerie ninja, though less unbalanced...) Bishops and psionics usually don't develop melee skills anyway. A fixed non-improving skill of 20 isn't that bad for an extended weapon. And since it potentially allows casting of a very powerful spell fairly early in the game, (unlimited casting, since items can be recharged by the old sell-them-and-buy-them-back trick) it's quite nice for early and mid-game use...