Dispelling a spell with one rnd duration seems less than likely, and constant can't be dispelled but for perhaps one rnd, so the increase in learning dispell magic escapes me.
Basically, a constant fieball is useless. So is a constant anybolt spell. A constant teleport could create a neat teleport booth/room/whatever, but it would be one way. An intersting trap could be a room with constant teleport to a set location and that location has a constant teleport back to the original location, creating a loop of endless teleports the PC is trapped in, unless a successful dispell (drop from teleport in a random location, i.e. GM fiat). The teleport would also have to have a radius. Quite a bit of spell mastery. Thank goodness for magic ritual.