Why mormons are 'always' asking you prying questions; two possible answers:
1) Mormons are sling-shot Quick to Judge. If they don't have the visual cues that differentiate people (dress, grooming), they'll ask you about your opinions about Romney/politics, SSM, abortion, etc etc etc. All so they can classify-categorize you.
2) people don't like to be considered superficial, that applies to non-mos as well.
But I can list dozens that do fit that category without breaking a sweat. These are the Mormons that go for the garment feel-up and are so stupid they think we don't know what they are doing.
It is not only Mormons who do that, but IMHO there is a bigger percentage of them who have no idea where the boundaries are regarding that sort of thing.
I think Mormons ask a lot of questions for the following reasons:
1) Like you said, they need information to judge and categorize you. They have trained to believe there is nothing wrong with assessing another's "worthiness" and are always trying to prove theirs.
2) Most of them have very little concept about what is and what isn't good manners in circles outside of Mormonism. So they don't realize how rude they are.
3) They have also been trained that they aren't entitled to normal boundaries so they don't know where normal boundaries are in their relationships with others.
4) They are all about trying to charm people into their church and they think they are "showing interest" rather than asking too many inappropriate questions.