AFAIK if the shaft of the motor is not supported by bearings at both ends internally, then you could easily use an outrigger bearing to support the outer end of the shaft if the shaft is being directly used as a friction drive or used to support a roller (or pulley/gear/etc). But if the motor shaft has bearings at both ends already, then you have to be sure to line up the outrigger bearing pretty precisely to the internal ones, and it has to be supported by the same frame as the motor itself, so that it can't move in relation to those bearings, or else you place extra stress on the spinning shaft that could break it along any stress riser it has (anywhere the shaft changes diameter, for any reason, especially).
It'd probably still be better to have the outrigger bearing than not to, if the shaft isn't hard enough and thick enough to prevent flexing under the drive loads, since those loads will do the same flexing to it anyway, and eventually snap it at the stress risers (like what happened with my fan motor on DayGlo Avenger way back when).
AFAIK it's almost (or is?) the same principle as a single-ended axle for a wheel.
