Interesting - I always found my 70-200 II to be 'awkward' on my 7D. It was too long indoors, and when I needed reach outdoors, it wasn't long enough...
Balance, and if your hands are large, somewhere for your pinky finger to rest. When using a heavy lens, I find the grip provides much better balance...
The closest I've come to that feeling is when I had a nice shot of a pair of roosting GBHs all lined up, and a grackle just flew up and landed right...