Brawn is the Kingmaker and I guess Kewis is hoping he'll do for him what Brawn did for Schumey and Button. If nothing else it'll be interesting.
Of more interest, Lewis is a clever guy and one of the most knowledgeable drivers on technical matters, with much formal training given to him by McLaren. His wee brain will bring a wealth of technical data to Brawn and it may disadvantage McLaren as much as it benefits Mercedes.
Conversely, now it's known he is going McLaren will seek to isolate him from future developments, upgrades and work on next seasons car which may impact on his title chances this year.