My 3 brothers and I grew up with various boats all used in salt and brackish water... None of us even today, too many years and many boats/motors later, runs an outboard without having one of those aboard. Compared to others we know who don't use them, I'd say we've had far fewer steering problems over the years (decades, that is!).
As for the "sand" concern, there should be an o-ring "wiper" built into the nut which will prevent any sand from getting in, just like those used on the retractable pins of the trim/tilt section of your outboard.
We don't use the whole "kit" you showed in the link though; as you should be able to buy just the stainless nut with a zirk grease fitting mounted onto it -- that's what we've always used, and it costs less too!
We all use Lucas Oil Marine Grease, but any "marine grease" would be fine. Just don't skimp and use standard automotive grease -- as you really want the extra corrosion protection the marine formula provides.
Good luck.
Old No7
