I have a NW, CO, and DL account. Why all three? Well, mainly due to foreign airlines, but wanting to accrue on US airlines.
I got my NW account first, and it is my primary Skyteam earning account.
Then, I moved to Taiwan, and flew Eva Airlines, which is/was a CO partner but not a NW partner. So, I got a CO account- figured it would be easier to use/top off than Eva's own FF program.
While in Taiwan, I flew China Air a couple of times- which is not a partner of NW or CO, but is/was of DL. So, there I got a DL account. Then when I got back to the US, there was that 25K miles for a low amount of money through Intercontinental, so I added to my balance that way.
How do I use them now? Well, my NW account is a "savings" account for an upcoming WBC trip back to Taiwan. DL is earn and burn based on whatever good promotions are out there right now (like the 5 partners for 5k miles) but without crediting flights to it. I mostly cleaned out CO a while back through a transfer to HH, but still have the account and add to it when there are free promotions (like sign up for RewardsNetwork and get 100 miles).
So there you have it- my main reasons for keeping two active are foreign airline mileage accrual + inability to transfer out (NW & DL both are not able to be transferred into anything else).