The most valuable answers I've seen on Quora were typically written by people who don't need (and probably wouldn't notice) small tipped amounts.
True, it's like if someone helps you with a favor, say helping you to move, then you turn around and offer them three bucks for their help, you're likely to get a rather negative reaction. Small tips are really not going to provide any more incentive for people who can provide valuable answers. But I'd love a way to attract attention to questions even if it meant having to put up significant money. Stack Overflow sort of does this with the reputation bounties.
Because the psychology of doing something for free vs. doing something for 10% of your usual hourly rate.
Plus answering on popular sites for making connections, building personal brand and even SEO.
Exactly. We see this as a new type of income source for many people. You provide cognitive value to the network and the network rewards you financially.