There's more than just the obvious to consider. Fuel usage is something else... endurance. Flying around <1,000 metric destroys your endurance. Also, at this altitude you hand over more too much to Lady Luck. Some might find this a 'fun' way to fly. Also.... there is a psychological thing about it too. The human mind, in Sims, likes to see the green out front aswell all the time. It doesn't like seeing just blue. Makes it less comfortable to get a feeling for orientation. I've often wondered why newer pilots always seem to fly around a same level. It's because they still see stuff on the ground outside the HUD. They don't have an altitude figure in their head. Just happens naturally. I expect newer Mig pilots fly lower because of the lower set seating in the pit aswell.