Because it only takes about 1 second for the rest of your team to focus and burst whoever you ult. If you force the ADC to build a QSS you've just delayed their full build considerably.
Even without his ult a farmed skarner is really scary. He's tanky and he sticks to his target like glue. He can force somebody out of a fight or force them to waste CC on him so that he can't screw the enemy ADC/mid. --- "SR'ing is having the RNG abuse you." Deshokun