Your point about private organisations paying for branding is interesting.

GNE need extra revenue. All companies are grateful for it.
If the GNE marketing team can sell the virtues of advertising and convince an organisation to pay for an all over advert they will.
That advert, just like any other form of commercial advertising doesnt guarantee success. Any marketeer will focus on the positives and promote their services in an effort to build revenue.
Whether that revenue is made via the traditional advertising on interior, the side panels or rear of buses (where revenue is supposedly falling) or to more adventurous marketing like the bus paints/wraps (who remembers the MCW deckers in the 80s and 90s with all over adverts? TDK, Radio Newcastle and Best Buy), bus tickets (I seem to rember Burger King were offering discounts on burgers in the mid-90s) or via the on-board monitors (that didnt last long!)
It is the world we live in.

However, what guarantees can the operator give that the bus will be working in an appropriate area?
It isnt necessarily a method that can be specifically targeted at an audience either.
And are the audience actually paying attention, if they are used to seeing a brightly coloured bus of some description everyday?
On my travels today I have seen buses of all sorts of descriptions, couldnt tell you about the adverts on the side of them though (there was one vehicle with a rear bonnet advert about an iron supplement, couldnt tell you its name though).