First I want to make this clear; no one in my guild is telling me that I need to roll a tank. I just feel guilty that some of them have to run ops on their tank alt instead of their dps. My main is a healer, so while I am not running them that often on my dps, I am running them on my main.
Why? If nobody is pressuring you to do something you aren't really interested in doing, why feel guilty about it? If your guild consistently has to have people switch to playing their alts to get the right composition for a group the solution isn't to have yet another person have a tank alt available. The solution is to get your guild to recruit people who's mains are actually tanks.