Dan Burkhart 

This has never been very popular, but it has been done over the years. Most notably, Maxi Car of years ago had them.
More recently, Curtis Odom used them in some of his hubs.
Now, if you read up on these things, you might think they would not be suitable for this application, but I am currently servicing a Curtis Odom hub set with somewhere between 30,000 and 40,000 kilometers on them, and they are still in like new condition. They still spin silky smooth with very low rolling resistance.
I provided this wheel set to a track rider that I sponsor, and he has more laps on our local velodrome than any other rider.
There has to be a reason why they have not been more widely used, but I'm not sure what that is.

Shown here with the ball cage in place. Yeah, it will get grease before I actually install it.

The cups are well polished and perfectly smooth.

canopus 

I would hazard a guess it has to do with the availbility of cartridge bearings vs the manufacturing of a special bearings and races and the labor to make it all smooth.polished and slick. Much easier for a manufacturer to lathe out a square surface in some aluminum and order a cartridge bearing from a catalog vs making a shell, forming it correctly, grinding and polishing it smooth, pressing it into a hub, order or making the custom inner race. making a custom bearing cage, then put it all together with that gorgeous looking axle....

But thats just mho..

mpetry912 

Great pics, Nice hubs !

Magneto bearings were designed for high rpm and relatively low loads. Perfect for a bicycle application.

I had a set of MaxiCar hubs new from Alex Singer and the rear hub was "grindy" - like a cheap hub. Jan Heine and George Gibbs both geeked on it and could not get it right.

that's a sample of one, I know. MaxiCars have a great reputation developed over time.

The 6003 sealed bearings used in Phil and other "sealed" hubs have a little higher rolling stiction, as there is drag imposed by seals and grease.

however they are inexpensive and plentiful, and they don't have alignment problems in the bearing - that problem is transferred to the hub shell. And performance models are available from Enduro bearings.

/markp

Dan Burkhart 

Well, I have found that preload adjustment has an extremely narrow sweet spot with these bearings. They are not exactly a radial bearing and not exactly angular contact.
The inner race is much like you would find in a deep groove radial bearing, but the outer race is a one sided cup like an angular contact, but there is no cone obviously as the ball cage is mounted to the deep grove inner race.
There is no shoulder on the axle to push the inner race against, so adjustment is much the same as an angular contact bearing with less room for error.

HillRider

In the OP you wondered why these bearings aren't used more often. I think in the above you answered your own question.