Nick,
I was also thrown by "past his prime". If he was a GREAT buck in his 3-6 years, and is weak now, i would INCOURAGE him to breed other does...
How many buck tags do you get? If its just 1, or even 2, your NOT going to do any help/harm by "culling" this buck. If its free range, you cant control which bucks "come" or "go" from your property anyways..plus most young bucks (1.5 year olds) disperse to other areas miles away from where they were born after their first year and a half or so...
Speaking of property, how big of an area do you hunt? That has alot to do with it.
And about the does, like mentioned, it is widely believed that they give up to 60% of the antler growing genes.