Fair points, and I completely agree! However, this was the best I could come up with giving the game's severe limitations on helmets (such as their inability to cover more than just the head).
Bevors also function as lower face armor, which is all I can make them do in this mod. I could rename plate armor to 'plate armor and bevor', but that would make it unable to cover the head. It would also no longer be a convenient add-on item to existing armor sets. Making an armor piece that only covers the neck is sadly impossible.
Ingame, kettle/sallet + bevor still has a small (2% i believe) chance of rolling no coverage at all, accounting for the gap left between the two. DF's coverage works a bit weird in that wearing two 80% coverage items results in a large chance of two layers of armor being hit, a smaller chance of only one being hit and a small chance of the attack bypassing both items completely. There is no way to make two non-100% coverage items give 100% coverage. Full helmets like the armet right now have the advantage of always giving full coverage and being somewhat less of a hassle.
I'm hoping Toady will expand a bit on the armor system at some point in the future. Once I get back into actually modding again I want to use DFhack to add various debuffs to armor pieces such as increased fatigue by weight, lowered agility for plate armor and reduced observation/weapon handling skill for full helmets that impair sight and breathing. They won't be massive debuffs, but will make medium-weight armor such as mail+brigandine+kettle have at least some edge over the heavier sets, as they historically had. I don't think DFhack will be able to cure the layering issues, but will at least take a poke at that too.