I would add shoulder protection on that list. The shoulder strap webbing will do the same damage as your crotch strap.
Why do you need to reinforce the elbows anyway?
And I find the knee reinforcement also redundant if you have good buoyancy.
I've carried double steel 120s one a Hogarthian rigged BP/W in my Seaskin trilam numerous times. I don't show any signs of wear on the shoulders. I would not bother with extra shoulder protection.
I would not bother with extra crotch protection, either, unless, one, you were going to be scootering, and two, you don't have a "soft" webbing crotch strap. Personally, all my crotch straps are either a soft webbing material, have a neoprene sleeve over the webbing, or are a split (Y-shaped) strap. I.e. for ME, I wouldn't bother with extra crotch protection, either.
I did get elbow and knee protection. Sometimes when I'm in a wreck it happens - a knee or an elbow will hit something. A bulkhead - the bottom of hatch, whatever. My buoyancy is not perfect. Neither is my awareness of exactly where my extremities are at all times. Sometimes I do screw up. And, I like the Kevlar knee pads that Seaskin offers because they ARE a little padded. So, when I'm on the boat and kneel down to look for something under my bench, the knee padding makes it a little more comfortable.