as a SID cannot use flight levels
Absolute crap. Zooker has beat me to some more sids, and I agree with his points.
Upton sids climb to FL60.
If airlines want constant climb departure routes, then there is no reason that they cannot climb to a flight level, even FL100, FL150 or higher. If this isnt possible due to traffic reasons, then its probably more often than not going to be a tactical solution anyway, and all the perceived benefits have already been lost. Lets not forget that most aircraft get a constant climb anyway due to the hardworking, underappreciated Atco's.
Benefits of climbing to a SID FL... you can set standard pressure on the ground! Now that would free up RT time and cockpit workload after departure wouldnt it!!!!