You're probably aware that many cities and neighborhoods now have ordinances and laws requiring us to pick up after our dogs. But even if you are in a more rural area, it is important to pick up his poop.
First of all, if it is your land and you don't want to clean up immediately, fine. That's your right. But you really have no right to leave anything behind on another's property. I hope you don't litter; and basically poo should be viewed the same as littering. Although it eventually breaks down, it does take a while. In the meantime, it will inconvenience the owner or whoever else may walk by.
Dog feces can carry zoonotic diseases (transfering from dog to human) and parasites such as Giardia. This can occur through direct contact or via flies and other pests. This is fairly common among kids. Think about it: a kid doesn't look where he's walking, steps in the poo, tries to clean it off, gets a little fecal matter on his hands which he doesn't wash very well, then a couple days later he's being treated for an intestinal parasite. And it all could have been avoided if someone wasn't being careless.
Make sure waste pick-up is part of your walking equipment. There are cool leash attachments and refills available that dispense poop bags. Many people like to recycle their plastic grocery and newspaper bags for this purpose. I have heard of some folks simply carrying around their pooper scooper and then flushing the waste when home.
Whatever method you use, make sure it is reliable; i.e. no holes in the bag. I even often carry extras to offer people who've forgotten theirs.