How useful or necessary is it to calibrate the battery on a MacBook?
If it is, should it be carried out when first purchased and does calibration of the battery mean it will also last longer?
There is almost no value in a user trying to "calibrate" a unibody MacBook's internal battery since it has multiple cells and the system handles this continually.
Older MacBook that were non-unibody construction with removable batteries did benefit periodic calibration runs to update the Mac's estimation of time remaining. Calibration didn't actually give you more power, just a more accurate estimate of the time remaining before that battery needs a recharge.
If you seek to extend the time between when you buy a battery (or computer) and the time when it needs to be replaced three things will help prolong the useful life of your battery.
In the end, you might pay between $100 and $150 for a new battery, but something that happens once every 3 to 4 years, Apple's new battery technology is far better than the old days where heavy users needed a new battery yearly and failures were far more common.