I noticed that nudity comes up in a few people's list of turn-off, mostly when it falls in the "useless" category.

I find there are two kinds of nudity that people care about: prurient, or meaningful. Sometimes a photo can be both, but the popularity of prurient photos on the Intertubes shows that it's easier to make than meaningful nudes.

The sad sack of nude is thus the non-prurient, meaningless one: just a naked girl, somewhere, not doing anything specific, nor representing much of significance.

Nobody really cares about it. It stimulates no sinful intent, and communicates nought.