It's not daft though as some aircraft would take more than 30 minutes to reach max altitude, also you could possibly meet enemies on your side of the lines high up - even German bombers came over high. Yes sometimes you could climb some of that on the way etc - but we have to set missions up to cope with most craft types and climb rates etc. I guess you could get all the climb rates and adjust mission climb outs based on those for each type etc, a job for a modder maybe. So many things, so little time.