Rubrail painting
The reasons you mentioned about not painting the rubrail are right on. Also, most rails are made of a rubber composite which makes it practically impossible to paint, along with the teflon seat ithat mounts to the boat. The first time you bump into something, it is chipped (or gouged) and is then a constant eyesore and just reminds everyone of the age of the boat.
I also don't mask directly up to the railing. Within 1 or 2 years, the constant flexing of the boat will begin to chip away some of the edges of the paint. The solution is to mask at least 1/8" away from the item. The result is a consistently clean paintline that wont be compromised.