The NW partnership is underrated, in my opinion. Sure, NW/CO combined is about the size of UA, but NW/CO is actually much better than UA out of my home airport (Kansas City). Two big benefits -

1) Two airlines occasionally will create a fare savings option. Every once in a while, a NW ticket or CO ticket is significantly cheaper than the going "major airlines" price. Having the two separate airlines to choose from essentially doubles the chance of getting a great deal.
2) Even when you don't get a better deal, having two airlines to choose from protects you when one of the airlines decides not to be competitive with the other airlines. If CO decides to price MCI-EWR higher than their competitors (a non-stop premium), I can always fly a NW connection.
3) NW/CO almost always gives me more raw options than UA. From MCI, I can get non-stop flights to only 2 UA hubs (DEN and ORD). I get non-stop flights to all 3 NW and all 3 CO hubs giving me many more options when flying to somewhere where I have to connect. Perhaps with the UA/US partnership UA will become better in this respect.

This is ignoring all of the differences in the FF program which other threads certainly cover.