Well supposedly only Demon Hunters can weild Glaives with as much precision and still be able to retain their agility, regardless of the weapon size. But anyway, Demon Hunters are melee Warlocks, more or less, like how Death Knights are melee frost mages or melee shadow priests/warlocks or how an Enha Shaman is a melee Elemental.
And I get your point regarding engineering, Mistweaver Monks use herbs, teas and and ancient remedies, but they aren't Herbalists.
The thing is, Tinkers, as you describe them and as they are requested, sound like having a limited concept. Mistweavers/Herbalists are part of a class, not the entire class. Even Death Knights and Demon Hunters have new concepts introduced to make them a class (like the vampiric Blood DKs or the far stretched yet wonderful soul eating Vengeance DHs turning into a monstrosity we haven't seen before). They even introduced new races to complete the themes, Pandaren and Aldrachi.
Tinkers as you describe them are fine for a spec but it doesn't sound enough for an entire class, they need more things to complete them.
I'm not arguing that they shouldn't exist, just discussing and saying my opinion on why they feel limited for a class or what more they could have to make them a class.