I think it's a little more serious; one should not drink the nonpotable water in the loos, so basically mouthwash is the only way to rinse without carting a bottle of water to the loo with you at the end of a flight - plus the mouthwash is niceto have handy prior to meeting your hosts/business colleagues at the end of a longhaul flight, and especially after a connecting journey post longhaul when the opportunity to brush may not be possible.

IMHO it is a valid concern, though I do not think the OP mentioned losing sleep over this particular aspect of the service.

Cheers.