Why is this still being debated? Why does it matter? Why am I not wearing any pants?
All questions that are really not important. The number one question should be: Do you enjoy getting together with your buds and play D&D?
For what it's worth, my players mix and match content. There's really not a different at all.
Here's why it matters:
Let's imagine one of the few D&D players left in Germany visits her favorite FLRPGS and notices a new supplement like, say 'Heroes of Shadow'. Having ignored the ten new introductory Essentials product, since she already has all of the 'classic' core books (and in German to boot), she's happy to see something that is apparently not an Essentials product.
However, when looking at the back cover she notices something that worries her:
For use with these Dungeons & Dragons Essentials products:
Heroes of the Fallen Lands
Heroes of the Forgotten Kingdoms
Rules Compendium
So, is it an Essentials product after all? Apparently so.
Since there's no mention about the book being useable without the listed Essentials books she asks the shop owner who tells her, that yes, the old books are no longer compatible and she will have to get the Essentials books to get any use out of 'Heroes of Shadow'.
Disappointed she puts the book back in the rack. Looking for something else to buy she notices something called Pathfinder. Curious she picks up one book and, wo and behold, it's German and it's apparently something like the successor of D&D.
That's the reality of where D&D 4e is today (in Germany).
In my opinion WotCs course of action actively discourages D&D players from continuing to support D&D 4e. Essentials hasn't helped to bring back players to D&D. It has made matters worse than ever by seeding confusion and uncertainty, initiating D&D's path back into obscurity.
To find out that 'Heroes of Shadow' is actually
mostly compatible with the original core books you have to either research the issue on the internet or you have to buy it and see for yourself despite what the back cover tells you. And that's just plain bad.
If 'classic' 4e and Essentials are as compatible as so many here claim, why isn't it acknowledged on the new products?! My opinion: It's not acknowledged for the same reason why it wasn't acknowledged after the switch from 3.0 to 3.5 happened: Some of the contained material might be difficult to use and/or require adaptation for someone who only has the old books (and it doesn't matter one bit if that is because it's a new edition or not).
Wasn't there in the 3e days some sales promotion where you could exchange your old books for the new ones? Something like this would have to happen for 4e, too, to (re)gain acceptance. And of course: The books must, must, must be translated into German again!