They may be gimped armies, but Grey Knights in particular have options (aka being taken as allies) that make them much more effective.
Oh, and Tau are absolutely 100% ass-raping nasty in the right hands. The local guy who plays them... *shivers*
And 'Crons are, if nothing else, hideously tough.
Also, most people don't interpret their WBB rule correctly. IIRC, the way it's phrased you can't be killed by sweeping advances or something like that. Which makes them much better in CC than they would appear.