Well I gather the derate numbers are certified for runway and climb gradient requirements...burning up valuable runway to that they can trend monitor an engine out to 25000 hrs before overhaul...
Passenger safety being number one of course....
Around here it looks like the 737s fly it to fence (8000 ft runway) then pull up..
The consensus being that after V1 has passed, your committed, lumbering down the runway waiting for V2, that if you lose an engine rotating at the fence(you planned accel/stop into the overrun) you now have the luxury of bringing up the good engine to max to get over those mountains ahead........lovely....