Plastic seats can be cleaned and sanitized more easily and faster than with the upholstered seats.
Bugs will not be able to burrow in and thrive as readily on plastic seats as they are able to do on upholstered seats.
The removal of bugs from plastic seats will be easier and more apparent to the rider.
Bugs will not be able to burrow in and thrive undetected on plastic seats as they do on upholstered seats. Removing them and all other substances will be easier and more readily evident.
I think the upholstered seats look good and add color, but I've had my share of wet seats. One cannot see with a casual glance if a seat is wet. but is easy to identify on a plain plastic seat.