You still have to babble the incantation. Just the deaf target isn't immune to suggestion because they still receive the command. It's a minor benefit of a ribbon ability that counters a minor fluff penalty of the spell (that doesnt matter 99.9% of the time). How often do you think this comes up to NOT just roll with a cool idea? Twice a campaign?
I swear some DM's look for any reason to say no...
One person's "any reason to say no" is another's "what stupid **** do you wan to pull this time?". Or "that's not how I see magic working in my world".
Any game has to have limitations and rules. Well, unless you're playing Calvinball.
I don't usually care about "slippery slope" arguments, but I do think this is one of them.
I always try to work with my players, but ruling has to make sense to the me as a DM. Sometimes that means saying no.