The 5D is a perfect wedding camera. High quality, excellent high ISO performance, enough MP to do anything you need, light weight enough to carry around all weekend long.
A 1DsMkII or MkIII is overkill in the MP department. More computer processing power required for editing, more storage needed for image storage and archives, etc. And to boot, its heavy to carry around all weekend with a L lens on it compares to the 5D, and at a substantial price in increase.
I am a firm believer to get the right tool for the job. A 1DsMkIII is a very nice camera, but a 5D is better matched for a wedding and event camera. And with the savings, you can better outfit your lighting, lenses, wireless, and computer.