My preferred program is DL -- but in a typical year, I'll fly NW, CO, AS, AF (all credited to DL), AA, UA, HP/US, WN and some European *A carriers.
I also stay at IC, Hilton, Starwood, Marriott, Hyatt and hotels that don't belong to chains.
There is no way I can get to where I need to go and stay with my primary program. I'm not going to change planes in SLC to fly SJC-LAS, for example -- that's just crazy.
Also, if my company travel department decides that AA is cheaper than DL on a flight, I'll have to fly AA.