For the first year the only benefit is more than 90 days phone support. But you pretty much get that anyway. Regardless of when you activate it you'll still only get 3 years from when the MB purchase was made, waiting a couple months wont make it last longer...
All AppleCare does is add 2 more years to your hardware support after the original year expires and extends phone support right through after the initial 90 days of complimentary phone support.
Since all else it equal, it's best to activate now. That way, you don't forget to activate it, and then potentially cause yourself some issues if you ever need to make a claim.
And I agree with Macky-Mac, it's too easy to forget to activate it in time and there have been plenty of sob-stories where folks who do forget despite having the AC in their hands, have been turned away from Apple. And it's nice to activate sooner just in case there are hiccups on Apple's end.
I know when I activated my iMac's extended AC, there were some hiccups and it took a couple of days to be fully noted with its extended coverage.