I always fly business, so I have short lines at check-in and extra baggage, more than I need. The price of business class tickets does not vary that much, short-notice travel is not much more expensive than booking months in advance - at least in my case. (I live in Asia.) I hardly ever upgrade to first class, because i don't like the extreme personal attention I get there. I feel comfortable in business.

Honestly, the main reason I WANT to belong in the top tier of a program is recognition. The attitude of the staff, the fact that I don't have to deal with hords of badly dressed, loud and obnoxious tourists around me, the feeling of having the right to complain, some sort of emotional leverage towards the airline, and not just be a sardine in a flying tin can. Call me a snob, but it is that which makes the travel experience worthwile. With top-tier membership, I feel I can WANT and HAVE things instead of having to ASK of BEG for them, or GET LUCKY.
I save a little money by occasionally upgrading to business, but in general its pure vanity. And the primordial collector's drive. And vanity.