Why is it not a good idea to climb higher than a level at which you would have a cruise weight-climb speed less than 500 ft/min?
Because ATC expects you to climb with at least 500 fpm when crossing to the next level. Otherwise you will block intermediate levels for an extended period of time.

If you are climbing 2000ft to the next available level for your direction of travel, that will take at most 4 minutes at 500fpm or more. This means the flight level you are crossing will need to be free of traffic from the opposite direction for four minutes to ensure legal separation is maintained. If you climb slower, ATC can't properly accomodate your climb.