Originally Posted by AgentPaper
Ah, but you're only looking at one side of the issue. The more you broaden what a class can do, the more you water down what classes mean as a whole. If you took a wizard to be anyone that can cast arcane spells, then you would remove sorcerers, warlocks, and any other kind of arcane caster from the game entirely. On the other hand, if you define the rogue more, then you allow for more classes to be made that would otherwise be redundant with the rogue. As those classes become more and more well-defined, you become more and more able to give those classes unique, interesting abilities that only make sense with the reduced scope of that class.
I just want to point out that "Arcane caster" is a concept MUCH broader in scope and potential mechanical implications than "Rogue". If anything, they needed to limit Wizard fluff further, and leave the rogue's fluff open.
There's only so many things you can give to a mundane rogue without making people scratch their heads and 'wtf' at you. Trying to make for example, a Swashbuckler, or a Scout, ends up making some very diluted classes that would have been better off just being a part of the Rogue in the first place. I mean this is the whole reason we got the name "Rogue" instead of "Thief", so you could play a larger range of concepts with the same class.
On the other extreme, the Wizard, whose main defining feature is "Arcane spellcaster who studies anything and everything" gets to do anything and everything that the developers think of, because arcane magic can do anything and everything. His spells known let him do more different things than any other class, and as more material gets released, that will only get worse.
If the Wizard was going to be the Generalist who knows everything, he should be closer to the Warlock, getting to learn every ritual spell out there, but non-rituals he gets restricted to lower level spells. Other classes would cover the specialist roles for specific types of magic (for example a Shapeshifter/Transmuter, and Abjurer, a Blaster, etc), which aren't nearly as flexible/studious/whatever, but are each much better at their chosen school of magic.