As for why, there's the usual talk about differing management styles, and so forth. Sponsorship matters could contribute, too. However, an insider did mention at Austin this year that Mobil was getting tired of seeing smoking McLaren-Hondas being hoisted onto a flatbed truck with Mobil and Esso stickers on them. As an aside, when it comes to Renault, Total might be cutting funding across the board, and might just be a technical partner, and not a sponsor. Red Bull seems to be ahead of the curve in all this, and with Adrian Newey once again interested in F1, things will be interesting.
And yes, Ron Dennis is McLaren. I agree with your post completely, OneEyeJack. I hope this isn't one of the "be careful what you wish for, because you just might get it" situations for those responsible.