The reason I suggested that keeping sent messages be optional is that
1. Not everyone wants to keep them.
2. Not every message is worth keeping.
3. If it is optional, then not everyOne will use it. And if it is not used by every member of the site, this saves on storage.
4. Option to save a message can be done on a message by message choice or have "Yes, save very message" option for the option.
Yes, I know that saved sent messages can be deleted. But that is simply another chore to be done. And if others are like me, I put off doing annoying picayune chores such as deleting messages.
As others have also suggested, having the original message one is replying to included would also be helpful.