Dear Carolyn: I never thought of looking at things like that! Do you really suggest going into a relationship thinking about the worst-case scenario?
-- Your Idea
Dear Idea: Of course it's brilliant. Cheez.
Seriously -- it's considered irresponsible to make any big decision without mentally walking yourself through the worst-case scenario. With any investment, you have to consider the risk. To avoid thinking this way with a relationship because doubts seem "mean" is, I think, the equivalent of choosing to shut off half your brain, arguably when you need it most.
Will asking yourself this question prevent all mistakes? No. All this can do is weed out the ones you already know are capable of taking prisoners and/or cheap shots. Almost everyone who gets out of a bad relationship will admit to having seen something was wrong long before choosing to get out. When there's an easy way to pre-empt a whole lot of bad decisions in one stroke, use it.