Although I'm not agreeing or disagreeing with you guys, you're missing the point of tying unlockables to Achievements.
Think of each class' base weapons. When you start playing the class, you only have those weapons. You get a feel for the class, understand how to use them. At that point, you should at least have Milestone 2. Then, with a little effort, Milestone 3 comes. The point of Achievements is to get you to understand how to play the class, before giving you the unlocks that are, whether or not they're upgrades, more advanced, with a few exceptions, such as the Blutsaugher, Ubersaw and KGB.
Backburner- Makes Pyro work all the harder to ambush targets.
Flare Gun- Pretty useless, actually. Nevermind.
Axtinguisher- Makes Pyro think a bit more about how he's going to ambush a target. If he keeps the Airblaster, he has little need for the Backburner if he has the Axtinguisher and is good, because he can Backburn the target then Axtinguish them, near instant kill.
Kritzkrieg- The mother of all situational unlocks. Seriously, you get a feel of when to Uber quite easily with the Ubercannon, but this requires a lot of thought to use. Imagine a first-time Medic using it. Yeah, not a good idea.
I could go on, but there's no point. If you really want the Unlocks right away, just use achievementhack.exe. Not hard.