One very important reason why it would be very silly (being polite) for a pro to use both systems, is that if one of his/her cameras breaks down, s/he cannot use the glass of one brand on the other camera which is of another brand.Really?... Serious sports shooting pros only have 2 cameras?..
Do not forget NPS and CPS work very differently from one country to another - very different regulations, set up and benefits. They might offer a replacement while they fix the broken one, and that replacement might be free, or it might not. But regardless, by the time you get it to Canon you have missed the shot(s) you were trying to get.
For landscape photographers that may not be a problem (I do not know as I rarely shoot landscapes), but if you are at a sports event and your long camera dies leaving you with only a wide angle camera, you are going to have problems, very big problems. If, in this scenario, you have the same brand of camera you can swap and change your lenses to your hearts content if one dies. Might not be ideal to only use the backup but at least you do not miss the shots and potentially your job or rep.
Now, there may be some very rich pro that can buy every lens for every brand to cover such an eventuality, but most can't. And even if they could who is going to carry that much stuff to an event!!
I do not care if you use Nikon, Canon, Sony or a paint and brush, but using both is just asking for trouble. It makes no sense.
i shot weddings with 4 different bodies hangin' off my neck with enough overlap that if a battery happened to go dead (they never did) then i could still cover the event
Same goes for back when we had to change film.
sarcasm aside (geez that's hard to do) how often do you break your gear that this would be a problem?
even if you HAD 2 of the same brand, swapping lenses is gonna cost you time and you're still gonna miss shots a bit like if you had 2 different systems.
I fail to see you making a valid point against using 2 different systems but as long as you convinced yourself into such a corner...