A big advantage of the spot metering method of achieving pretty accurate ETTR is the facility to select in the scene what what is the brightest 'relevant' part, composition wise.
For example, you have a heavily backlit subject against foliage and blue sky through the gaps in the foliage. If you really want the blue sky, you would choose that as the brightest part of the image, then increase exposure by 3 stops. The main subject would then appear too dark and would have to be lifted in post processing with consequent noise.
However, a more sensible approach might be to sacrifice the blue sky, take a spot meter reading of, say, the white collar of the dress the subject is wearing and make the 3 stop increase from there.
Such an approach was hampered in previous models of Canon DSLRs because they had only a +/- 2 stop scale in the viewfinder. I think this has now been increased to +/- 3 stops in recent models, which is ideal for this spot metering method. No need for maths to get in the way of the artistic process. One simply turns the exposure dial till the needle is on +3, eye still glued to the viewfinder.
