Less dish is better for a dished rear wheel. The less a wheel is dished, the less difference there is between the tension of drive-side and non-drive-side spokes, thus making for a stronger and more durable wheel. Plus, on wheels with freewheels, less dish means less exposed drive-side axle at risk of being bent by torque.
So, why are so many wheels dished more than necessary? By this I mean having a 4-5mm gap between the chain and the drive-side dropout, when the chain is on the smallest cog.
By moving a spacer or some washers from the drive-side axle to the non-drive-side axle, I can decrease the dish of the wheel and substantially up the spoke tension of the non-drive-side spokes, resulting in a rear wheel that should be a lot more durable.
What I'm wondering is, why is this common in bikes? I've seen it especially frequently in older wheels with five- or six-speed freewheels. Which is even more problematic, because there is a second danger of dishing on rear wheels with freewheels - excess drive-side axle means greater chance of bending the axle.
Is this just because bike companies were lazy? Or a hub that came from the factory spaced for a 7-speed freewheel wasn't re-spaced for a six-speed freewheel? (I've also seen this on even older bikes with five-speed freewheels, back when there were only five-speed freewheels, so this can't be the only reason).