Having recently worked alongside both FAA and ICAO controllers this topic was frequently discussed.
The reason either "maintain"(FAA) or "altitude" (UK) is inserted before the level is to prevent confusion. If you omit either of these words you end up with a clearance like this -
"climb to wun zero thousand feet"
...which can easily be misinterpreted to mean "climb 21'000ft"
Unfortunately I don't think ICAO phraseology addresses this problem, and unless your state files a difference, you are stuck with using the slightly less safe ICAO phraseology.