Yes, there is a reason to think that a 7D image will outperform a 5D MkII image if you crop the 5D image to the same field of view as the 7D image.
The 5D MkII has a 21.1 MP sensor, the 7D has an 18 MP sensor. Let's say we put a 600mm lens on each camera and take a shot. To get the same FOV in each case we have to crop the 5D MkII image down to APS-C size. Full frame has 2.56x the area of APS-C, so if we crop the 5D MkII image (21.1 MP) down to APS-C size we end up with only 8.25 MP.
So there's the comparison. an 8.25 MP cropped image from a 5D MkII, or an 18 MP image from a 7D. Which would you pick?
That's the reason that a lens can be said to have more "reach" on an APS-C body. We should really be taking about angle of view not focal length because it's the angle of view that changes, not the focal length. However people are used to thinking in terms of focal length when it comes to magnification.
If I said that a lens has a FOV of 4.3 degees on a full frame camera but only 2.7 degrees on a crop sensor body, most people wouldn't have a clue as to what that would mean. On the other hand if I said the lens had the FOV of a 300mm lens on a full frame body but a crop sensor body it had the same field of view as a 480mm lens on a full frame body, most photographers would have a gut feeling for what that means.
Here's more on Full frame vs. Crop sensor.http://www.bobatkins.com/photography/digital/full_frame_vs_aps-c.htmlhttp://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html
The bottom line is that if your lens isn't long enough for you to get the framing you want with full frame, you're better off with a crop sensor body than cropping a full frame image. If you have the luxury of either using a 300mm lens on a 7D or a 500mm lens on a 5D MkII, the 500mm lens on the full frame body will give you a better image. If you only have a 300mm lens, the 300 on the 7D will give you a better image than an equivalently cropped image on the 5D MkII.