Mercurius
Legend
This is probably worthy of a thread in itself, but it seems to me that there are people who think that Essentials is an addition to "classic" 4E and there are people who think that Essentials is a replacement for "classic" 4E.
For those in the former group, Essentials is just another bunch of options. The operative verb is not "switching", but "adopting".
It isn't an either/or thing; in fact, it is somewhere between the two. But I agree that one doesn't have to replace "classic" 4E with Essentials and that one can view the bulk of it as additions ala Martial Power, but there are also more wide-ranging systemic effects, like magic item rarities and a bunch of little tweaks that I'm not aware of but have heard about.
I also agree with whoever said that the Rules Compendium is a "must have." I have used that little book with greater frequency at the table than any other game book in memory. It is a gem.