I'm gonna deny that it's just a broadening of the "class bucket". I think originally class and role were pretty much tied together to the point where a class, for the most part, defined what role you were playing... now having a fighter in the party doesn't necessarily mean you have a defender. Again, I see this transition as pretty much making your class obsolete.
Ok, so what precisely do you mean by 'obsolete'? Yes, it seems like classes will no longer be tied to one role. That statement is a valid one. The following statement, that this "makes your class obsolete" - what does that mean?
Are you saying that each class now no longer has any concept it is tied to? That's obviously not true. Power source, mechanics, flavor, all of those elements are still there. The fighter is still, generally, a skilled warrior who uses martial training to be a big hero. Usually by getting in their face - some fighters are tanks that draw enemy fire, others just put them in the ground.
How is the Slayer different from the Barbarian? The Barbarian flies into berzerker rages, drawing from the primal power of the land as he destroys his enemies. He is much more mobile, tends to be less heavily armored, and thrives on tearing through the ranks of enemies.
How is the Avenger different from the Paladin? Flavor-wise, the Avenger is more of a secret agent of the faith, while the Paladin is the front line. The Paladin might go save a village and spread the good word, while the Avenger hunts creatures in their own element of the darkness, his deeds never mentioned in the light of day. Mechanically, of course, they are completely different, with one a lightly armored and mobile striker and the other a heavily armored defender who heals his allies.
What difference between the Archer Slayer and Archer Ranger? Well, the Slayer is tougher, more likely to be heavily armored, and relies on a few well placed shots and his own skill with the bow to bring down his foes. The Ranger, as noted, has a few elements tied more to nature and the wild - and even outside of those, is very much a more mobile, lightly armored character, who tends to fill the air with arrows as he brings down his enemies, and takes them out by finding their weaknesses and taking advantage of them (represented by Hunter's Quarry.)
Now, it is worth noting? That one can ignore all the above. You can get pretty far with building against type or finding your own flavor that fits your concept. Just like you could in 3rd Edition, for the most part, and possible before that. But the classes still have definite amounts of flavor tied to them conceptually, and definite mechanical differences that crop up in actual play, and I don't see how anything in Essentials will cause the class system to just fall apart or breakdown or grow obsolete.