This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Roller head bearings provided an advantage that is not directly
connected with rollers. However, compound ball and plain bearings
have recently replaced rollers as is described in the item on "Indexed
Steering". The main advantage of rollers was that they have two
bearings in one that is important because the bearing must accomplish
two functions. The problem of the head bearing is so obscure, that
until recently, no one had taken into account that head bearing is
subjected different motion than is apparent.
The bearing serves as a hinge about which the front wheel assembly
rotates, but it also absorbs another motion, and this is the problem.
As the bicycle rolls over roughness, the fork absorbs shock mostly by
flex just above and below the fork crown that makes it rotate fore and
aft about a horizontal axis. The motion can be seen by sighting over
the handle bars to the front hub while rocking the bicycle fore and
aft with the front brake locked. This is what occurs when rolling
down a paved road but with much smaller amplitude.
The angles through which the fork crown swivels are extremely small in
contrast to the relative motion at the hub because the distance
between the hub and the fork crown is large. This motion is not in
itself damaging to the bearing because it is only a small misalignment
that cup and cone ball bearings absorb easily. The damage occurs when
these small motions occur when there are no steering motions to
replenish lubricant while the bearing balls fret in place. Fretting
breaks down the lubricant film on which the balls normally roll and
without which they weld to the races and tear out tiny particles.
Because rollers could not absorb these motions, they were equipped
with spherical backing plates hat could move in that direction. This
was the contribution rollers made before they were replaced by ball
bearings that had this same feature. Balls, in contrast to rollers,
stay in alignment and do not bind up from sliding off center as
rollers often did.
See item on "Indexed Steering".