What's really going on here seems to be an internal political battle for resources and a management desire to reduce development costs by designing, building and supporting one database engine for all Microsoft products.
It makes sense from their managements point of view and I can understand why Microsoft manage OUTSIDE of the Exchange Product Team keeps bringing this up. (And they're the only ones I hear internally talking about it as a 'good' idea.)
It's about control and specialization. ESE is tweaked to support Exchange and AD is doing a superior job at it. I manage 15,000 users across 130 stores and I don't want more complexity. Backups are easy and relatively fast. I have a bunch HA options in 2007 and some very creative HA/DR options in 2010. I want the Exchange Team to be in control of our database engine.... period!
I hope the Exchange Team continues to win the internal struggle. OBTW, I'm not implying the SQL Team cares one way or another. This is a management team cost issue not a custumer service issue to them. ESE is still the customer service solution IMHO.
Cheers