I have used the codeshare between NW and CO depending on where I want the miles to be credited. Last year I used NW until I qualified for Silver Elite WPerks, then in October '99 switched to CO. This year I am going for the Gold with CO OnePass, but have flown some NW and HP flights.
Flight availablity, fare and destination determines which airline, I use, but the miles all go to one FFer account in a given period.
I think it is a win-win arrangement.