I don't wear rings. I'm not a big jewelry person. I can't stand having something on my fingers constantly. Plus, the few times I tried wearing rings, I'd leave them at home, wash my hands in a public bathroom and suddenly go "OH NO, I LOST MY RINGS, I LOST MY -- you dope, you put them away for safe keeping at home."

My boyfriend is under instructions not to buy me rings. If he was going to cheat on me, a ring certainly wouldn't prevent that. I trust him and know he can say "No, thank you, I'm with someone" without a chunk of gold from Zales to prompt him.
*ETA: He does messy construction work anyway. So I wouldn't expect him to wear a ring for that reason, too.
Regarding her ring, can you imagine the noise that would have been made if they had chosen to get engaged, and she never had and never planned to wear an engagement ring? Ho boy, I can imagine the headlines now - all accusing him of being all sorts of things.
Huh? I don't get it! If a woman doesn't wear an engagement ring what does her fiance get accused of? 
Did you mean accusing her? I can see that!
I'm not LEMon, but I'd imagine headlines insinuating that either William is too cheap/doesn't care enough to buy a ring or that the family doesn't trust a commoner with family jewels, etc.
As far as the issue here, I agree it's completely between them. I will admit that I love the idea of a wedding ring on my (hypothetical) husband, and I'd be disappointed if he didn't want one at all ever...but that's my thing and I'm sure there'd be some middle ground that made us both happy if I end up wearing a guy who doesn't like rings. If Kate's cool with it, that's all that matters.
What exactly is the middle ground, though ? (aside from "it's on my finger" or "it won't be on my finger") I don't mean that in a snarky way, I'm genuinely curious