DK's start at 55th, and you'll be 57 or so when you leave their area.
That's basically RAF for a team, just by starting them.

You have all the flightpaths for the old world, although you'll need to explore if you care for that achievement.

They're a strong combination to consider.
Not sure if you'll have as much success in arena with 5x anything, as you'll have 4x shammies plus 1x dedicated healer (not you).
But as far as battleground go, or world pvp's.. 5x DK's are intimidating.

I personally prefer shammies or druids, but DK's are a good option.