I use multiple programmes in all alliances. You never know when your needs will change and you get all of their special offers as well.

I have a system going now where I post discounted C to my european cards and discounted economy to my USA based programmes. They often still award 100% where you get only 25% or no miles with with european programmes while the european programmes often award 200% for discounted C.

Some US programmes have great additional non air partners where you can also accumulate substantial miles. I buy all electronics now in the USA online for in store pick-up and get 5 miles for every $1 when I am in the states. This is great as I can just stop in a store and pick up purchases I have ordered from my European home before departing.

Another thing is that often having the FF card today can save time at check-in as you can just swipe your card for check-in. Even booking direct with carrier is eased by having a FF account as you save time in the booking process.

I book online using the carriers own FF to get the booking and check-in bonuses (500-1000 miles each time)and then get a new boarding card at the airport (at lounge or gate) and change to my account where I want to accumulate points or to get correct status boarding card. (or occasionally upgrade as this does give you contact with gate agent)