Comment 1 for bug 505732

After some more thinking and code inspection (mostly regarding blocktype plugins), here is my conclusion:

Completely removing a plugin from the database would involve wiping out all user data associated with that plugin (block instances, etc). This is probably not desirable. User data should be preserved so that the plugin can be installed again at a later date. This basically what the "disable" feature does - effectively removes the plugin from the site as far as users are concerned, but keeps all their data.

So, this feature would likely be used mostly, if not only, by developers working on new plugins, who need to start from scratch. Not very high priority, since they would typically know how to hack SQL as needed. Would still be a nice option, however.