They have to pick something.

And if there are known and repeatable variations in performance at different times of day (and, yes, there absolutely are; I've run the numbers on DOT data covering a large span of time and documented it for another job) then you absolutely should account for it in planning. Moreover, more aggressive scheduling means generally lower labor costs, assuming the schedule can hold. The airline's goal has to be to schedule the bare minimum it can reliably operate. And UA's on-time numbers keep ticking up.