Trust me, more than half the stuff I carry around is in support of my laptop, and yes there's more than 10 pounds there. It's a moot point, though, because:

1) a bike heavy on one side really isn't that bad, you don't notice it.

2) a good pannier won't fall off, get wet inside, get caught in the spokes, etc

3) backpacks really do suck for long distances, and people who sing backpack praises haven't likely tried using decent panniers for a month worth of 30-mile round trip commutes.