1. You should not compare low AP leathers to high AP ones. Wyrm sure is good but the AP puts it to heavier tact vest/Riot gear category. But again, most leather users will be low AP ones.
2. Sure heartbreaker and bison have some use, but given that the best defence is to kill everything or just move out of range it is hardly comparable. You craft Bison for 1 dungeon but still ...
3. G. Siphoner, one specific very niche build, and it is very questionable anyway.
4. Hopper is decent, have some uses, but I rarely see anyone picking it over rathound.