but when faced with a paper'n'pen couldn't produce anything coherent
You don't use paper and pen to program in real life, so asking people to do so in an interview is not very useful. We test peoples' technical aptitude by first giving them a little programming task. It's the same task for everyone, they have an hour and a half to get as much done as possible, and we don't expect people to complete it. They have access to the interweb and all the online docs.
Then we go through their code with them, and talk about other relevant stuff.