It's just a bargaining tool for extracting more money out of McLaren, and/or Hamilton getting to keep the trophies rather than staying in McLaren's rather large sideboard.
As much as I'd love to see what sort of a merry-go-round we might end up with should it happen, it just doesn't make sense.
If Hamilton was going to end up at Red Bull, then he'd have signed for Red Bull this year, replacing Webber, who's contract was also up. If he was moving to Ferrari, then he'd be moving to Ferrari this year, replacing Massa, who's contract is also up. If Hamilton was to join Renault, then he would be joining this year, to replace Grosjean, who will surely be released after this year (depending on his reliability in the remaining races). He wouldn't be joining Mercedes as a stop-gap to joining Red Bull, Ferrari or Renault, when all three have/had available spaces for 2013.
And on what we've seen since 1974, with the exception of the first half of 2009, no driver in their right mind would be joining Mercedes/Brawn/BAR/Tyrrell expecting to have a car capable of winning races consistently.
It's just a tool for a better deal from McLaren, who apparently will have to tighten their budget next year, as there's rumours that as well as having to pay for Mercedes engines, they may also lose their Vodafore sponsorship.
mercedes have so far wanted to remain all german.
Not according to Mercedes they didn't - they said that was just a coincidence. How true that is I don't know, but they're not exactly renowned for having German drivers in other series - even in DTM they've had drivers from a multitude of countries in their top teams, notably Gary Paffett and Paul di Resta.