an application would be made to grab all of its allotted RAM, even if it left most of it subsequently unused. Another application might be memory starved, but would be unable to utilize the free memory "owned" by another application.

While an application could not beneficially utilize another application's heap, it could certainly destroy it, typically by inadvertently writing to a nonsense address. An application accidentally treating a fragment of text or image, or an unassigned location as a pointer could easily overwrite the code