It depends. The bearing currents are often low enough that a bigger bearing is often cheaper and easier. Plastics used to support live parts or for insulation in higher voltage systems have to be flame rated. Flame rated plastics are neither cheap nor easy to work with. The bearing current issue is generally associated with high reliability, long life applications or Harbor Freight / Walmart motors for fixed speed appliances that have been cheapened up so much that the now tiny bearings will no longer tolerate inverter drives.
Ungrounded moving parts also generate static charges. When the static charge accumulates to the point where it jumps across the nearest gap, it can create ground bounces or other spurious signals and general interference. One of our famous experiments had an isolated 120VDC motor being run as a generator, picking up charge from the belt like a Van de Graaff, and creating loud, 2-3mm arcs every few seconds.
If you were watching the big game and the blender being used to make margaritas was arcing to ground and pixelating everything every few seconds would you be happy?
About half of the cheaper, smaller motors I work with have one metal end cap and one plastic end cap. The first for cost (a simple stamping) and the second because of a complicated shape that integrates some electrical isolation. The grounded (drive end) bearing is big and handles up to about 8-10mA of bearing current. The isolated (fan end) bearing is only half the size and sees no bearing current.