Switching power supplies regulate best in the upper half of their range. Pulling 40 watts from an 85 watt power supply may give you some ripple. That said, we have a gigantic capacitor in our Macs (called a "battery"
) that smooths any lows out pretty well, and all the internal voltages are supplied by internal voltage regulators that chop out the ripple before it can get to internal components. As long as it charges (i.e., the low parts of the ripple don't fall below the gate voltage of the charging circuit), you aren't harming your Macbook.
Oh, why you may want to get the 85 watt supply for your Macbook as your extra power supply -- the 60W supply gets *hot* when the Macbook is under load, and the battery charges extremely slowly. It's clear to me at least that the 60W supply is pretty marginal for the Macbook when it's chugging both CPU's hard and playing a DVD too, not only do you have the howling fan, but the power cube gets so hot that it becomes hard to pick up. So if you really push your Macbook hard, as vs. just use it for web browsing and word processing, the 85w may provide some "peace of mind" that your power supply isn't going to melt through the top of your computer desk