I think that in your personals, like with a person's last words, it should show you what you said to them. Pretty simple. 

This doesn't really seem needed. If you can't remember your whisper it must not of been too important.

I guess there could be some use with recalling old whispers but I'm not sure it would really add anything to the game.

It's more of a little addition etc. Not complex or anything. More just for flair. 

My only concern would be the increase in personals. For instance: If I was to shoot and kill someone, leave a whisper and they had the last words enabled I would be receiving 3 personals for a kill.

This just seems like too much to me. Personally if this was an optional addition, I would never use it.

I dont really see the need to it tbh.

It's been used elsewhere. The only benefit is really for memory's sake etc. Other than that, I don't think it's needed.

I have an absolutely terrible memory, but even then I see this as a cosmetic change thats not needed at all.