A few days ago, Random mentioned Gmail and the controversy over whether the concept of deleting e-mail is obsolete. It's an interesting exercise in product management.
With Gmail, Google has tried to address the real requirements relating to e-mail. The ability to delete e-mail messages is not itself a requirement. Deleting e-mail messages is one possible solution that addresses two important requirements:
1. Ease of finding old messages of interest.
2. Scalability (not running out of storage space).
Deleting e-mail messages is actually a poor way to satisfy these requirements, since it helps only marginally with finding old messages of interest and places burden on the user to keep a tidy inbox.
Gmail makes searching old e-mail messages as easy as a typical Google web search, and provides so much storage space that it's difficult to run out. Gmail thereby satisfies both requirements without burdening the user with e-mail deletion.