>>>five digit number becomes complicated and confusing by
>>>the addition of one more digit...which is always either a 5 or a 0?!
All I can say is that you have never flown into LHR or LGW at peak hours. What is easier to recall, 132.475 or 3247?
And the proof of the pudding is the number of missed frequency changes, especially while in 'foreign' regions. I count about six or ten missed frequencies every day. Thats six or ten opportunities for extra ATC stress and a potential incident. And for what reason? Because some desk-jockey has only half a brain and did not ask users what they want.
Besides, when you are on a very busy frequency, with no room to get a word in edgeways, what is simpler and shorter to say - 132decimal475 or 3247 ?? So ATC are struggling to control the numbers of aircraft in thier sector, because of unwieldy phraseology. KISS wins every time, while complexity leads to incidents and accidents.