I'm not sure about the cropping argument either. Taking 36mp and cropping down to give the same lens multiplication factor as your 7D APS-C sensor reduces the effective pixel count to 14mp v the 18mp of the 7D...
Or put it another way with a 300mm lens on full frame, cropping to give an effective 600mm lens leaves you with only 9mp
With the same 300mm lens on the 7D the native crop immediately makes it equivalent to 480mm lens so cropping to give an effective 600mm leaves you with 14mp
For wildlife photography, I think crop sensors have got to be the way to go unless you've got very deep pockets for very long (& heavy) lenses.