I actually have a problem where I want to heal in BGs but I am in dps spec out in the world. I queue as healer, and if I forget to switch from dps to healer before I enter the BG my game lags so hard that I can barely do anything. I assume it has something to do with some addon but have not taken the time to investigate. Essentially, the manual switch from dps to heal messes things up.
I do know that if you queue for two roles (ie tank and dps) that you can switch freely during prep in BGs, but if you queue for only one you can only play that role.