[whpalmer4]

No question it would be one less thing to do in the system maintenance department if OF pruned its backups automatically (assuming it was implemented in a way that makes sense for your workflow). But it is a zero-sum game — is that really the most important change that could be made to OF for you, or for anyone? Disks fill up, it's the nature of computing life, and OF would just be saving you from a relatively trivial task. Your workaround of backing up to another disk is slightly increasing robustness, as doing backups to the same disk where the data lives leaves you vulnerable to a failure of the disk.

Just put in a repeating action to remind you to prune the backups occasionally. It only takes a few seconds. Or go the Hazel route as others have suggested, which costs some money but likely can be put to additional uses. Or wait for this request to become the one with the most votes :-)