I think it's something to do with some of the more verbose ATCOs not being satisfied with simply saying "roger". I do agree that there are a lot of unncessary calls made.
However: Many a time pilots call on frequency and do not report their current flight level. The good book (MATS part1) says that in this instance, the flight level must be confirmed by ATC and an acknowledgement received (or something along those lines).
In my opinion;
"ABC123, maintain FL330"
"Maintain FL330, ABC123"
satisfies this requirement.
There are a whole load of other reasons for saying "maintain" on first contact. Not least to make sure that there is absolutely no ambiguity when pilots check in with;
"London, ABC123 FL330 - Can we expect to be FLxxx by xxx today?"
Believe it or not, some pilots take the response "affirm" to mean "you are so cleared".
Sorry to hear that you are so annoyed by idle R/T chatter. Puts you off your G&T does it ?