I think I "mostly" understand your request, but am not sure if you are looking for Reply-To specifically, or whether auto-populating the From field will do. When appropriate, I use [UserInfo.Membership.Email,System] in the From field. (Actually, I assign that to an Action variable if I need it more than once, then use [emlFrom,Action] or something similar.) Most email clients will use the From field when Reply-To is not specified.
Regardless, there are scenarios where I would like to override the Reply-To with another address; so overall I agree with your request.
A word of caution though (and you may already know this), some servers, clients, and other security software may reject or block an email where the From and Reply-To differ. I personally see this as over-zealous spam control, but it does happen.
Good luck with the request!