Personally, I think the best reason to use euphemisms like these is when you have a complex issue, you need to illustrate two opposing behaviors or viewpoints, and you don't really want to discuss your specific viewpoint.
So for example, if you have an issue that requires a lot of explanation, and it is related to the fact that the OP is a Methodist, but her SIL is a Lutheran...it might make a lot of sense for the OP to instead identify as an apple, and call her SIL a pear. Because if the OP isn't really interested in talking about Methodist beliefs, even in a polite and general way, the best way to avoid that is to just not identify herself as Methodist.
Since everything posted on the forum is fair game, it might be desirable to obscure highly personal details like that, even if the risk of anyone turning it into a debate about the detail itself is quite low.
I do think that if you have an etiquette question that involves a detail you don't really want to discuss, the best thing to do is to simply leave it out of the question. But sometimes, that detail is so integral to the situation that leaving it out would just not be possible. Usually, my personal answer to that situation is to just not post at all, but I can understand why someone might just try to be vague about what religions (or whatever) they are talking about.
Plus, I sometimes think that using euphemisms can serve to broaden the discussion. Talking about Purples and Oranges allows people to fill in the blanks however they like, even if it has no resemblance to the situation the OP was talking about.