Well before I started with FT, I was very agnostic about flying out of SEA:

HP, WN, AS

Eventually got elite status on AS.

Then I learned more, and started playing the game, buying miles (or converting) on arbitrage. So now it looks like this:

AA, UA, WN, US, BA, VX, VS*, IB*, AS, (still elite on AS)

Right now I actually have more *A miles in UA/US than AS miles (thanks to me cashing out about 200,000 miles in the past year on AS, plus 45,000 AA miles).

I'm pretty agnostic on who I will fly in the US, thanks to AS elite status. I will get reasonable room in coach on AA/DL/AS, occasional upgrades on DL, more than occasional upgrades on AS. But I've actually flown UA and US in the past year without elite status and lived to tell the tale.

*miles through Hilton