even in cases where the sentence is extremely long and complex, following the rules of grammar tend to make the sentence more clear than unclear.

I couldn't fail to disagree with you less.

I don't like to have to kill nobody without they ain't no chance of no gold in it for me.


One descriptivist complaint about prescriptivists is, as the Dragon says, that they think that descriptivists believe there are no rules. Descriptivists recognize that there are rules but the ones that the prescriptivists recognize don't come anywhere near describing the language that people use. Prescriptivist rules are fine if you are writing a style manual for some publication but if they can't describe the simple phrasal verb examples I used above or the one that zmjezhd mentioned they don't describe the language.