Except that Clerics, Druids, Bards, possibly Paladins are ALSO "magic users". If they all have spells... they all use magic. Now you might then make the distinction between Arcane magic-users, and non-Arcane magic-users... but then we get right back into the Power Source issue that so many people decried.
Power sources weren't really decried from a conceptual standpoint; as the Jester noted, they were mostly decried because they either didn't do anything (Martial, Arcane, and Primal didn't really have shared mechanics, just a shared theme) or did too little (Divine had Channel Divinity, but every class's CD did something different) and aside from a few paragon paths nothing really keyed off them.
If 5e uses AD&D's super-class groupings, then "power source" would actually have an impact, because that gives you class inheritance (paladins and rangers in 1e were "as a fighter, except the following," which lets classes share mechanics and/or a class chassis without too much redundancy in the rules) and the ability to have "X group only" items, specialties, etc. (instead of having the Necromancer theme say "this specialty is only for wizards, sorcerers, wu jen, [insert arcane class here], ...." it can just say "this is a magic-user theme).
And what's lame about it is that Warlocks aren't really Arcane anyway. Warlocks are and have always been much closer to Clerics than Wizards... because their magic is granted to them by some much more powerful entity. The only reason they've been called 'arcane' is because there's this nebulous energy called 'arcane energy' that D&D has created and yet has never really defined... and have just randomly assigned certain classes to it. How is the magic granted by a devil the same as the magic you get by using hand gestures, incantations and bit and pieces of random crap? Or the same magic as playing music? To me, they aren't similar at all. So to call a Warlock's magic 'arcane magic' is just hand-waving the fluff because they don't want to call a devil a source of 'divine' magic, since it's always been "the gods" who had a moratorium on that term.
Arcane magic has at least one pretty clear distinction from divine magic, at least pre-4e: arcane magic is something you own, divine magic is something you borrow. If a cleric or paladin pisses off his deity, or a druid breaks his oaths, goodbye magic; if a warlock pisses off his infernal patron, too bad, he was given his power and no take-backs. This was blurred in 4e once divine casters no longer could lose their powers for breaching their code of conduct and warlocks had pact boons rather than just making one agreement in their backstory, but given the revival of older flavor I'm guessing we'll see paladin and cleric codes again, whether you like it or not.
But hell... (pun intended)... Asmodeus is the king of the devils as well as a god in 4E. Which means a cleric and a warlock can get their powers from exactly the same person. If that doesn't mean clerics and warlocks should actually be put in the same category rather than warlocks and wizards, I don't know what would.
Fey pact warlocks... okay, maybe their magic is arcane. Infernal warlocks? Divine magic all the way. Which tells me creating a "super-group" of all arcane casters is kinda useless and is actually antithetical to its own fluff.
Personally, I hope Asmodeus goes back to being a non-divine Power again; making every powerful being a deity changed a lot flavor-wise--Asmodeus no longer has a true form in Nessus, Archdevils/Demon Princes/Archomentals now have to deal with worshipers and such, you don't have the difference between an organized evil religion and a smaller evil cult, etc.--and, I feel, made things less distinct and interesting.