People hate the idea because they simply don't understand the argument of why a Tinker should be in WoW.
Reason #1: The Tinker shares the same pedigree as other expansion classes (Monks, DKs, and DHs). The Tinker originally was a hero in WC3 just like all the previous expansion class entrants. Currently, the Tinker and the Goblin Alchemist are the only 2 remaining WC3 heroes whose abilities don't exist in the class lineup (and they don't exist in the professions either). So essentially Blizzard has a skeleton of a class just from available WC3 abilities alone.
Reason #2: Goblins, Gnomes, and Mechagnomes have no class that reflects their racial background. When you start one of those three races, you begin in an area surrounded by technology. This works against the immersion of the race, and causes people to only choose these races for irony purposes. This problem is amplified when you look at Mekkatorque, Gazlowe, and Gallywix, the leaders of these races. All three of these characters pilot mechs, and that alongside with the nature of these three races indicates that their use of technology is pretty common.
So let's say you fight against Mekkatorque in BoD and you think him being inside a robot suit is pretty cool and you want to make a Gnome character just like him. What class can you pick? Oh yeah that's right, you have NOTHING to choose from that even remotely matches that concept. If you like BFA style Anduin, you can roll a Paladin. If you like Sylvanas, you can roll a Hunter. If you like Thrall, you can roll a Shaman. If you dig Gnomes and Goblins in mechs (a concept shown over and over among those races since MoP), your SoL.
Reason #3: A Tinker class won't take away from existing class concepts. Necromancers would take abilities from Death Knights. Dark Rangers would take abilities away from Hunters. The Tinker takes abilities away from no one, and despite the common argument here, they wouldn't take away from engineering either. Why? Because other classes that have abilities similar to professions haven't harmed the professions in any way. When Hunters got grenades, no grenades from engineering were removed. When Monks and Rogues got the ability to create potions that they can give to other players, Alchemy still got new potions and elixirs. Tinkers have their OWN abilities from WC3 and HotS, and NONE of those abilities exist in engineering.
Reason #4: It will actually HELP Enigneering. The common argument I see around here is that a Tinker class would take away from Engineering. Nothing could be further from the truth. First and foremost, professions don't compete with classes, they're designed to work alongside classes. So a player who rolled a Tinker can also pick up the engineering profession, and that would just add to their immersion experience. How would engineering benefit from a Tinker class? Simple, a technology based class can give engineering a new class to build stuff for. For example, there could be craftable engineering items that change the appearance of a Tinker's mech, or their summons, sort of like glyphs. They could also make exclusive armor like headgear or weaponry that matches the tinker aesthetic. If Tinkers use guns, that would be another class that engineers can sell guns to.
Speaking of engineering...
Reason #5: A profession isn't a replacement for a class. Engineering isn't designed to replace the Tinker class. This is proven by the fact that none of the Tinker's abilities exist in the profession, and Blizzard has shown that there is a set of Goblins and Gnomes who can take the concept of engineering to an entirely different level that the profession can never reach. For those that say that a hero using engineering as a power makes no sense, consider Iron Man from the Avengers. The Tinker is essentially the medieval version of Iron Man, and the profession is a hobbyist who cobbles stuff together to make trinkets for sale. That's the difference, and that's why saying that the profession covers the Tinker makes no sense.