Orks actually do have a lot of transport options, and the battlewagon is just obscenely good for it's points. Huge transport capacity, AV 14 in the front and open topped. That means that if you put, say, burnaz in one, they can all shoot out. Can you say "15 flame templates"? Bosses can take them as dedicated transports as well, and bosses can be troops. You can also put deathrollaz on those for some brutal ramming action.
Hell, I just realized you could squeeze 9 battlewagons with deathrollaz into 2000 points, 6 with small boss units, 3 with burnaz. Ouch!