This will work great.
I've run a number of cards off 2xmolex->1x6pin and never had any problem. I suppose YMMV.
I guess you never actually tried them, judging from your theoretical "OMG IT WONT WORK". Like I said, I own over 25 of them. Worst case scenario, the plug at the adapter's female end gets a little warm, but nothing horribly dangerous. I've daisy chained them before too, but load balancing needs to be kept in mind then. The PCI-E 6 pin is grossly under specced. This is why the spec doubled the load on the 8 pin by only adding 2 more grounds. The 12v wires remain at 3.
A little googling around revealed the maximum power spec from a molex is 11 amps per pin, so at 12 volts, the 4 pin molex is specced for 132 watts (This doesn't mean if a molex line has 4 plugs you should run over 500 watts through it. This is 132 watts MAX for the entire line). PCI-E has triple the 12v and even more grounds. You should be able to draw 30 amps, or 360 watts, with little problem.
If you need proof to what happens when you overload a line of molexes, this is what happens. This was with ONLY 2 radeon 5850.