I address this issue, and show a home-made tool that helps in the
adjustment, in my article at
http://sheldonbrown.com/
My theory for why new hubs come adjusted too tight is that it is easier to true the wheel when there's no play in the hub, so it's better to make the adjustment _after_ the wheel is built up.
Customers will easily notice a wheel with play in it, but will rarely notice a too-tight bearing until it self destructs, somewhere down the line.
Jobst Brandt's assertions about all bearings needing preload is standard wisdom in the contemporary bearing industry--but bicycles commonly use cup-and-cone bearings, a type ignored in, a least the most highly regarded contemporary text on bearings.
I'm not convinced that the assertion that only a couple of balls will take the load if the bearing doesn't have preload applies to cup-and-cone bearings, though it is obviously true of radial contact bearings.
I've seen a LOT of hubs over the years in normal service, and the ones that come in badly pitted seem, disproportionally, to be those that are adjusted tight.
Sheldon "Not Convinced About Preload" Brown
Newtonville, Massachusetts
+--------------------------------------------+
| My daughter Tova and I will appear in |
| Gilbert & Sullivan's _The Grand Duke_ |
| May 4, 5 & at the |
| M.I.T. Student Center, Cambridge, Mass. |
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Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772, 617-244-1040, FAX 617-244-1041
http://harriscyclery.com
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Useful articles about bicycles and cycling
http://sheldonbrown.com