I have a 40-year old nylon backpack that's still in great shape, and I've never sprayed it with anything. (It's an antique external frame - the retro look gets lots of comments!).

My nylon canoe pack is much younger, and also isn't showing any real signs of wear (or UV damage).

The UV protectant probably won't hurt things, but it might not help either. It could make your pack more slippery (sort of like putting Armor-All on your car's steering wheel, not a good idea). I like to adjust trim on my canoe by moving the position of my pack forward and backward, so for me, a more slippery pack that shifts position when I don't want it to would not be a desirable outcome.

If it ain't broke, don't fix it?