1 thing to remember, 'breathable' and 'waterproof' are simply not able to be used together in an absolute sense. If the fabrics were as breathable as they say then in all reality there would be no need for 'pit zips'. The reason they are there is to get rid of heat and moisture that the fabric cannot. If it is made more breathable to let that kind of heat and moisture out, then it lets too much moisture in, hence the constant paradoxical battle to obtain maximum breathability and maximum waterproofing.
That said, the use of a good goretex shell is a great addition to downpours for short periods, i think the attraction to cheap ponchos is that it covers a larger area and gives you use of your hands underneath for cameras, phones and other things.. besides they also double up as impromptu stroller tents! ;o)