The 'guy who uses a sword and magic' is a pretty classic archetype, and needs a dedicated class to be viable.
In 3.x, yes, you needed a dedicated class or PrC to make the martial/magical combo work, because casters just didn't multiclass efficiently. In 4e, you just have to get stats to align, and MCing or Hybriding delivers, you don't have a 'caster level' to worry about. An Artful Rogue/Fey Warlock, for instance, or a Fighter/Shaman, or an Eladrin Tactical Warlord/Wizard/Wizard of the Spiral Tower, or a Dragonborn Paladin/Dragon Sorcerer (ooh...).
Of course, it's Essentials that's mixing sources, and that may be because there is no intent to include multiclassing or hybrids in Essentials, in which case, yes, as in 3.5, dedicated 'gish' classes become necessary.
The unarmored, intellectual divine caster most definitely has a place, and the Cleric does not do that well; maybe another leader instead of a controller like the invoker would have been better, but again, it was hardly pure gap-filling.
The Invoker gets chainmail. And, really, armored or not isn't that big a deal anymore. The distinction between the Cleric and Invoker is quite subtle, if you had an Invoker concept, before, you could have done it as a Cleric with no problem, you'd have been a leader rather than controller, but you could have covered the /concept/.
Until WotC decided they could have builds of the same class fill different roles (which I don't think was a good idea; it's probably the only thing in the Essentials line that I really don't like), the various things a 3.x druid could do had to be split up into different classes or tossed by the wayside. Melee druids were really good in 3.x; having nothing viable to convert such a character to in 4e would have been bad.
Splitting up classes in that sense was more about 3.x classes being radically overpowered. You still can't do CoDzilla, and that a good thing. Letting builds fill different roles isn't such a bad thing, though, for that reason. It's still 'splitting a class up,' but if it lets it do something cool it could do in the past in a balanced way, there's no harm.
Psychic Warriors are almost as old as D&D psionics. The Battlemind is just the 4e version.
D&D Psionics goes back to the 1e PH. It's always been a horrid, grafted-on bit of sci-fi crap, though, so, whatever...