I care...most of the time. I ride a Honda ST1300, considering its size I think I get fantastic mileage. Last year I averaged 5.48L/100km. I try to stay to 120 on the highway, having my current MPG displayed on the dash (I realize it's not 100% accurate) helps me be more aware of my mileage. It helps keep me to a more respectable speed.
Taking it easy does make a huge difference, on a trip south last year I got my best mileage by getting off the superslab and just puttering along for a day (4.6l/100km). I'm not really good at doing that however day to day when commuting to work or just going for a ride around here. With a 7.7 gallon tank, managing over 500km per tank is easy, I usually need a break before the STeed needs a sip.
When I go down to the BRP or dragon or anywhere where there are good twisties, I really don't care that much. But even then, the worst I get is just a hair over 6L/100km. What affects my mileage even more than riding hard is the temp. Dec and Jan I averaged about 6.4L/100km. I imagine if I eliminated all cold weather riding, I'd be averaging around 5 no problem.
Although I wasn't keeping track back then, I don't think I got much better mileage commuting with my GS500 on the highway. But the comfort and range of the ST is night and day to anything else I've ever been on. I started keeping track a couple of years ago with an app on my phone and although I'm anal about tracking this info, I mainly do it just because I like to keep track and know what's going on with the bike.