A rubber tube can act as an excellent flex coupling to counteract any misalignment between the motor and to whatever it's coupled, but the function of the felt washer is to introduce some resistance to keep the motor from bouncing back and forth within its backlash tolerence. I bought about three dozen really nice Olympus brushless DC motors with beautifully made 1000:1 planetary gear heads off of eBay that I intended to use to build up a Laserium capable projector (and always intended to use felt washers if needed), but I broke my hand and Jon came to my rescue & loaned me a projector for the duration of the gig, and yes I used felt washers with the original Laserium portescap motors where needed.
"There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso