As for the Maintain FL***, I wonder if it came about because of errors from the pilots and a controller missing a readback....
IE - BAW356 is level at FL180 - Controller: BAW356 Decend to 5000 feet
Pilot - Roger Descend 5000 ft BAW356 (missing the "to" part of the transmission and controller missing the correct readback). The pilot then leaves FL 180 and stops at 13,000ft, descending only 5000 feet.
Maintain 5000 feet reduces the possibility of listening and readback error.
I think I remember the correct phraseology at some point being Descend and maintain FL*** or climb and maintain FL***, but that drew complaints because the descend or climb part made it far to obvious. IE, if I am at FL180 and given clearance to FL270 I am not going to decend to get there am I? Which makes the UK version of it seem a bit silly too doesn't it? Wich is better? As long as no one bumps they both seem fine to me!