What happens if they still don't pass after their extra two attempts? Do they get to go to special school?...Hang on, they'd actually have to make places available at them. 
DH teaches children with SEN. He was so chuffed and excited when some of them managed to get a Level 3 in their SATs last year. They had worked so hard (as had he and anyone else involved in teaching this little group).
It's got nothing to do with standards or rigour or discipline. Some of the stuff KS2 children need to know for these test, most adults in this country have no clue about. (I actually doubt that David Cameron or his clever advisers would be able to answer all of the questions correctly, particularly for the SPaG paper. It's obsolete knowledge...unless the majority of my class have the ambition to follow in my footsteps and do a degree in linguistics, which I doubt.) All it will achieve is that more primary schools teach to the tests in Year 6. For those children, who struggle, Year 7 will then continue to be a string of interventions, removed from their peer group and the chance of actually getting settled.
Most secondary schools set for core subjects, so there isn't an issue of the clever children being "dragged down" by the less able. They don't drag anyone down in my mixed-ability class, either. I teach to the top and then we break things down for anyone, who doesn't get it. My most able make accelerated progress,...so do my least able. It's my lower middle, who I need to keep an eye on.