What are the chances of your getting back into the race bunch after you've stopped to fix a puncture? Race tyres aren't well known for being easy to fit or remove, so even with CO2 you'll be 5 minutes or more off the back.
If you rely on CO2, either you are going to have to carry extra cartridges over the top of the number of tubes you've got, or you are going to end up walking home or phoning for help at some stage.
With a pump rather than one CO2 per tube, you have the following advantages...
You can put air in the punctured tube to find where the hole is.
You can correct for any tyre fitting errors.
Failing to fit your gas supply to the valve properly isn't a problem.
Its worth carrying instant patches for those times when you failed to find the cause of the problem (and there *will* be some).
You can pump up a tube you've mended with an instant patch
Also, as lazyfatgit pointed out, inner tubes are somewhat porous to CO2 and at some stage you'll miss the start of a ride because you forgot to swap CO2 for air and were in too much of a hurry on the morning of the ride.