1) Many hardware vendors, especially storage, will give you trouble or refuse to support their gear if you're running an OS they don't bless
2) It can make business continuity easier; you'll find more people who claim RHEL/CentOS support than Debian (though Ubuntu is pretty wide-spread these days).
The rest are pretty minor. Some C level tools want to feel like they're magically protected by vendor support.
The reality is I've pretty much always had to do my own diagnostics, RCA and come up with a resolution. If there's no patch in a RHEL blessed kernel but there is in mainline, I'd just as soon pull it in and patch it myself, which is way easier to do IMO using something like Debian, or write a patch myself if necessary.
So I'd say if you're having to deal with a lot of hardware support it might be worth it just to get less pushback but if you're mostly cloud based or just have a few machines go with whatever you think rocks (unless you suck, but you would never realize that :P)
At one shop we all wanted to use Debian but the vendor only "supported" suse/rhel etc.. so we went so far as to modify uname and whatnot to pretend we were a redhat shop ;)