Actually, they only do if I "remind" them. (I like to think I'm not just herding them into a pen....) They can only breed once a week. But once you've got dragons of a particular color, if you want more dragons of that color, breeding them is a lot easier than trying to pick up eggs from the cave. And some types of dragons can only be produced by breeding, e.g. "geodes." You need to breed two stone dragons, or two "pebble" dragons (those are the large light green ones) or stone x pebble. All my stone and pebble dragons are female, and don't seem interested in experimenting with new um... gender identities. I was kind of hoping this most recent egg would be a medium green one and would hatch into a male, but it was a dark green one (vines) and I didn't get to keep it anyway.
What I think would be more fun would be a sort of garden that would change depending on what you started with and which things were next to each other. E.g. some kinds of flowers might attract butterflies, some kinds of plants help other plants grow, etc. And you could get cross-pollination, and share the seeds with your friends. But I guess more people like clicking like mad to try to "steal" eggs for big fierce dragons.