I agree that flexibility is important when possible. However, in my case, I barely requalify for EXP each year and I've decided that the perks I receive as top tier with AA outweigh the advantages of being mid/low-tier in two or more programs.

I can make this work because I am able to choose the airline I fly in virtually all cases, plus I am able to opt out of locations not served by AA. If either/both of these variable were to change, I would probably rethink my strategy and go for mid-tier with 2 or 3 carriers.