Even if you have status on UA, I think LAXRuss' point is valid: you can only upgrade on UA with UA miles. Transferring MR points to CO now seems a relatively secure way to get them into the ultimately merged CO and UA FF program. I can't find any other way to get MR points into either the UA or AA programs; if you upgrade from business to first, not coach to business, then UA and AA are practically speaking the only US-based options for international travel: the other carriers (including DL, US, and CO itself) have nothing more than business class on offer.

So, I expect I will also transfer a lot of MR points to CO.