For event photographers 12Mp is very good, the subjects are not likely to feature in huge prints often. Most who know their way around Sony files can pick them up at high ISO and the regularly quoted figure for the a7r is a ceiling of 6400, and it's really good below 4000. The a7r looks very decent at 6400 being the point, and not much mainstream photography is not covered with an f2 lens.
If you want (or need) versatility the still medium size pixels of the a7r accommodate you better - much better in fact, for crops and tonal gradation. There is a good strong case for the a7s but I feel it will sell on greater 'tactile utility' - silent shutter, better EVF display, faster writing, more assured focusing. And of course dual stills/video use.
Had it been 16Mp, more buyers would go for it, no need to pause.
And looking at Ashwin's images a little up, the truism that low Mp cameras need better lenses looks to hold true, making the a7s a little more expensive than it appears for top results.