I guess I'm just trying to see the logic in not bouncing.
As I said previously for the most part it is a waste of time with forged from address, etc.
You have more or less said it your self, 'spammers aren't going to waste time' and going through their lists removing bounced emails (assuming the bounced email ever reaches them) to clean up their list offers no financial benefit. So do you thing they will do that, no, I don't think so either.
If everyone bounced spam, just think of the increased email traffic, not to mention you may get your bounced email back as undeliverable.
I would recommend adding another entry to the Origins of Spam section - sbl-xbl.spamhaus.org - this catches most of my spam and I just set it to flag for deletion when I process my mail - no extra effort or time required.
As you have found that when you bounce spam it is outgoing email which will be sent by your email client and will be scanned as normal (an overhead).