But sometimes it does mean an unresponsive system. All an application has to do is allocate a bunch of RAM (or spin the CPU on a number of threads >= cores) and the system slows to a crawl. It can't really kill those apps unless I ask it to, because work might be lost.

On iPhone, if apps save as they go, excessive usage can be solved with a kill -9 when I press the home button. Relaunching the app will take a few seconds and is transparent; I don't have to fight with the malfunctioning instance.