I've paid extra for K fares on multiple occasions. Most of the time it's when I'm traveling with my wife and want to use miles/SWU to get up front. I booked her a ticket to Hartford over the summer and paid extra then used miles to confirm her in first. We're traveling to Boston in May with our 2 year old, and I paid extra for K fares to use a SWU for son/myself and miles for my wife on a separate PNR.

After the JFK-SFO/LAX/SEA announcement of no complimentary upgrades, I'll probably be saving SWU's to use on that route and buying K fares to get up front. I route through JFK a lot heading to the east coast. Already have my solo return from Boston booked through JFK using a SWU, and paida bout $100 extra for the higher fare to help get me up front.

I've never paid the higher fare just to get ahead of the line in the upgrade list. It's always when I'm using a SWU or miles for a confirmed upgrade (or wait list on SFO-JFK)