then why replace it with a 70-200mm f2.8 L is usm then
Because the 55mm isn't really long enough for some of the shots. In actual fact, I got the 70-200 for a wedding. It is not a good lens, and I would never recommend it to anyone. Mine is far too soft and has focus issues, despite being back to Canon. It's going back again soon....
lets be honest, the quality of kit does make a difference as does the ability to use it and the knowledge of where to use it. i like the full fame 5d but it comes at a cost, with the loss of focal length compared for a crop sensor but it also has advantages