Heroku has good reason for this. The number of test apps that people create to try the system and then never shut down is probably quite high. They don't want to be running 10,000x512MB worth of apps that aren't serving any traffic. However, that also means that if you have a low-traffic (but real app) running on their system, the dyno is likely to get shut down during the times where you have no traffic.
A 1/2 size dyno would allow people to have a cheaper dyno that wouldn't be shut down.