Disagree that it is irrelevant. Assuming that we accept that a fair house price is 125 times the monthly rent then we can work out the fair price and know if we are getting good or bad value. Very relevant in any purchase IMO.
I agree that there is a fundamental equivalence between rental income and fair price, however 125 times rent would equate to a rental yield of over 10%. I'd be more inclined to put the ratio at 200 times monthly rent, which is about 6%.
Either way, ratios of 500-700 are plain crazy, simple as that.