AI goes so way to reflect altitudes impact on energy state. If you want to slow down you can knock points off speed and then climb which will further reduce your speed. Diving will quicky take you up to max speed. So you can trade altitude for speed and vice versa.
Some planes have lower thrust so struggle to gain speed and climb others have the ability to climb and increase speed because of their thrust (power to weight ration).
A lot of AI missions we play involve a ground attack element so are at quite low altitude. The game we played on Saturday one of the fighter groups started too high and stayed up too high, this allowed their bombers to get bounced before they could get down and support and when they did come down they came down in a power dive and some of them got picked off by fighters that had climed up and turned into their dive.
The game is very like WW2 or Vietnam type missions in feel.