That's an absurd comparison, and you know it. Yes, a melee ranger is going to use the melee ranger powers. The same dichotomy exists for a number of other builds - Str vs Wis Clerics, Str vs Cha Paladins, Con vs Cha Warlocks. On the other hand, you have a number of builds that blend those two - I've seen paladins who use both Str and Cha, and so forth. You could probably even do the same with a Ranger - characters can focus on two ability scores, after all. I just doubt most people have any concept that encourages them to do so.
But guess what? Those are all a completely different situation than wizards choosing between implements or a rogue choosing between Artful Dodger and Brutal Scoundrel.
I played a Bugbear Rogue up to level 7. The character was dedicated to damage dealing, with a high strength and a heavy focus on Brutal Scoundrel. And yet - my most used power was Bait and Switch, designed for use for Artful Dodgers, since it was still extremely effective for the character and great for moving around combat. Most of my utility powers were movement based ones (Tumble, etc), even though mobility is supposed to be the 'domain' of the Artful Dodger.
I have a Level 3 Star Warlock in LFR, with a maxed out Constitution. His powers are split 50/50 between Star Pact and Infernal Pact. Yes, he isn't going to take any Fey Pact powers with his 10 Charisma - but there are still a variety of choices for him to make when he chooses his powers.
Guess what? Both characters, extremely effective. Often hands-down the most effective of the table, despite apparently playing builds you feel removes their "ability to function."
Heck, your comparison isn't even actually accurate for Rangers. I'm looking at their Power list, and many of their powers can be chosen for Melee or Ranged. I don't think there is a single level where they are forced into a single specific power based on build - there is almost always a choice between two powers, and sometimes more. And I imagine Martial Power will vastly expand that more.
I ran into this same argument once before regarding Fighters and the idea that their weapon choice defines what powers they take 100%. Even at the levels where different weapons gain bonuses for different powers, I still find myself considering a variety of options depending on what my characters wants out of their powers - whether they want to hit multiple enemies, or keep enemies from escaping them, or debuff their enemies, or just do the most possible damage.
Now, if your point is that a character's goals and concepts will define them utterly... well, sure. If a character wants to play a rogue designed around always having combat advantage, and only takes the powers that specifically help with that, then yes, they have a very direct path. But builds really aren't that restrictive - claiming that the choices you make at level one define 90% of the character, for the rest of their career, is completely absurd.
I mean... ok, I can see your concern. I can see why you might think the builds are so restrictive. But in practice, they very much aren't. Yes, a strength-based ranger will only have 2-3 choices of power at each level, but they aren't locked into a single choice. From the discussion given by the writers of Martial Power, while many of the powers it will add will be for the Beast path, several more will be for any rangers, so I suspect this will bring the choices up to 4-5 at each level - and more for other Martial classes. I can certainly understand being concerned about this danger, but I think 4E avoids it - I have been specifically impressed by how hard it is to choose the best power to take at any given level, even when playing multi-stat classes where my choices are already limited.
I'd be happy to provide examples, since while I get your concern, I absolutely don't see any support for it in the actual rules.
