~~~~ (what it expands) is too long, and (I think) redundant. For timestamps you can always see the history (I think).

I can't make you do it, but timestamps following a post allow others to immediately see when a post was written without having to dig through revision history.

Just present me a good reason and (probably) I will use the longlong version.

You can customize your signature as you like, but it's mainly as a benefit to others to include a timestamp with it, especially when you're starting a new discussion section. That way someone can look at the talk page and immediately see when a topic was discussed.