Anemones will live long lives with or without fish . There presence or lack there of will not affect that either way. The only functional benefit would be detritus being cleared away a little faster than would have happened without a clown brushing through the tentacles. Anemones are quite efficient at removing unwanted debris from their mouth and surrounding disc. The ammonia would be negligable, zooxanthellae uses only minute quatities along with other nitrogenous compounds that would easily be gained from simple water ingestion. If anything, the excess ammonia and other wastes produced by the hosting fish would be a negative impact, not a possitive one.
As for protecting the anemone, it really depends on the attacker and it's size. Certain clown species will quite agressively defend their host but a greater percentage would just as easily turn tail.
In the long run their is no clear benefit to the anemone. The rest is mere speculation and at best anecdotal.