Those are for the most part useful/needed to them to make wiser choices in how they adapt and streamline the series.

It's an insurance policy if he doesn't manage to publish those books before they have to produce this part of the story, but if that happens they will have to write those seasons themselves following pointers/outlines, and that's not quite the same job as starting from a finished novel - and will lack Martin's "revisions" to his storytelling which he does as he writes (and we already know his plans on paper for the series didn't always pass the test when he actually wrote the final novels... what he gave them as "spoilers" could end up being told rather differently in his novels). In the end, the last season(s) would be "fan fiction" developped by pros, and based off Martin's plans for main plot points rather than an adaptation of his finished story.

I get the feeling Martin and his publisher have decided that the odds of this happening are very, very low (readers of Martin have a somewhat less optimistic opinion). This is a situation where everyone would lose. There could be a drop in ratings as many choose to wait for the book before they continue watching the series, and Martin/Bantam would see his novel massively spoiled and his readers discovering his ending in a completely other form before they get to read the real thing. As for readers, it would really suck that after waiting so long, so little about the final book feels fresh, because we've seen a streamlined version on TV (not to mention the series would create all kinds of expectations/anticipations about each storyline that would be quite distracting when reading the novel later. It could well end up with "the book is better and might have been an awesome read, but too bad thanks to the series the suspense/surprise/tragic elements have so little impact left).