I really don't see any significant benefit from enabling or disabling this 'remove from sever when deleted from deleted items'  because  I have tried adding a Gmail account using POP3, then I have deleted account. However, I saw that the messages on the remote mail server are still there. I have tried deleting some messages from inbox into deleted items while account is still added in WLM, then I deleted from deleted items. Finally, I still saw no messages deleted from remote mail server of gmail account.