I had a thought last night about Williams and Mercedes. I believe that Mercedes have put a lot of time and effort into their V6 engines and so the F1 grid might reasonably expect the Mercedes V6s to be pretty decent engines, especially when you consider their current V8 engines. So obviously Williams have signed up because they want a good, reliable engine and Mercedes are probably cheaper than Renault. For the 84-87 seasons, Williams used Honda engines, so they, like McLaren, have historical links.
What if Williams decided to change to Mercedes to change engines and be a bit more competitive but be ready to move to Honda if McLaren achieve good things in 2015. Williams would have used Mercedes for 2 years by the time the 2016 season came around and I wonder if there's a break clause that allows Williams to leave Mercedes for another engine supplier after 2 or 3 years?
Could this be Frank Williams hedging his bets on Mercedes as a reliable option for 2-3 years and see how Honda do on their return?