In Berachoth (3a) a Beraitha states that there are 3 reasons not to enter ruins - that it is unseemly, that walls might collapse, and that unsavory characters might lie in wait and harm the person. The Gemara goes on to discuss whether three reasons are really needed, and it justifies each reason.
For some reason, the objections raised against the third reason are so great that the Gemara has to limit the scope of the Beraitha. The Gemara finally adds, "I Ba'ith Eimah" (if you want, you could say, ie., an alternative explanation is) there's one person all alone, in restored ruins in a remote field, so there's no concern about it collapsing, and there's no suspicion that he will do anything unseemly because it's so remote the odds of finding a prostitute are zero, but unsavory characters do lie in wait and might attack.
Why? Why does the Gemara feel compelled to scrutinize and justify each reason, and why does it object so strongly to the last justification that it has to come up with this isolated scenario? Why can't it just accept that these are all valid reasons not to enter ruins? In fact, I'm actually surprised it didn't go into a round of different Amoraim each adding a new reason.
EDIT It has been suggested in the comments that perhaps the Gemara sees a practical difference (Nafka Minah) in the reasons. Is that so?