If I were Eric Boullier and I had plenty of time on my hands, I would have asked Frank Williams if I could borrow Valterri Bottas for the weekend.
It seems inevitable that Bottas will race for Williams, but the team has been very reluctant to drop one of their regular drivers and put Bottas behind the wheel (if I were Frank Williams, Bruno Senna would have been shown the door months ago). Lending him to Lotus for the weekend would give them the opportunity to assess him as a driver and give them that little bit more extra information about his performance.
At the same time, Lotus stand to get something out of it, too: they need a competitive driver. Right now, they're locked in the heat of battle with McLaren and Ferrari over second, third and fourth place in the World Constructors' Championship. They need someone who can help secure a good points haul, because otherwise they run the risk of losing touch with McLaren and being overrun by Ferrari. Bottas, while untested in race conditions, is highly-regarded by Williams, and Nico Hulkenberg - Bottas' predecessor as Williams' Next Big Thing - is still very well-rated in the paddock. He also has more experience that any other driver not currently engaged to race with the 2012 cars and tyres.
So Williams get a chance to test their new boy, while Lotus get someone who represents the best chance at securing points.