I'd tend to look at block hours per aircraft per day as a more accurate single measure of utilisation (utilisation as "intensity of sweating the asset") than number of sectors, which is entirely dependent on the particular route structure. Maybe an airline can generate more revenue from 8 short sectors than 6 long sectors, maybe not. Some of its direct operating costs are definitely going to be higher flying 8 rather than 6 sectors, though. I don't think it's possible to simplistically say "8 sectors good, 6 sectors bad."
Yes, I'd agree. You can't just make a general statement that 6 sectors per day is bad utilisation. A lot of Aer Lingus's routes from Cork are to European destinations which obviously are longer flights. Ryanair's Cork based aircraft operates 10 rotations per day. That sounds impressive until you consider that 8 of the 10 are short Cork to Dublin flights, and the other 2 are relatively short flights to/from Gatwick. The utilisation of the Cork based Ryanair aircraft is actually worse than any of the Cork based Aer lingus aircraft. It only leaves Cork at 07.40 and arrives back for the night at 21.40, and it spends a lot more time on the ground during the day than any of the Aer Lingus aircraft as well. You really do have to look at block hours and not the number of sectors to see how good the utilisation is.