Mike
As ever your observations are always challenging to the status quo.
So it makes me think why do I always measure the size of every commercial job.
I measure it because I want to know the size (to make sure I haven't been told porkies) and because it gives me a STARTING point in working out what I want to charge.
By measuring the size and computing what the price SHOUD be at my standard "per metre" price I can then adjust to factor in all those other things like access, level of soiling, time of day, furniture, water pressure at taps, distance to toilets etc etc, to come up with "the price".
It also allows me to consider more carefully how much i want to push for the job or will be happy to let the opportunity pass me by. IN other words if the price they want it doing for works out at some ridiculously cheap per metre price then for me at least, I can know why I may pass the opportunity by. If you can afford to, chase profitable jobs rather than turnover jobs.
In other words by measuring the area it is simply a standard to work from.
Finally, measuring to some extent is a piece of showmanship and theatre in that you are seen to being objective in your pricing (even if you are not).
Then when you send your quote in with perhaps it broken down on an area-by-area basis, together with say some acknowledgement of the issues to be overcome in the cleaning the (potential) client is more likely to understand how you came up with the figures that you did come up with.
Rog