Ticket systems are great and I use them for pretty much everything I do, and places like github make it very easy to use them even for your small hobby projects

Ticket systems are great and I use them for pretty much everything I do, and places like github make it very easy to use them even for your small hobby projects. However I still think it's valuable to add TODO and the likes to your code, especially when working on open source, since the issue tracker might not be around forever, you switch platforms, project gets forked, some part reused in another project. It also makes it much easier to get into a new code base, just like comments in general. Even at work we do this even though using tickets for everything is mandatory. I like it. And in case we open source some stuff folks won't have access to our internal ticket system obviously, so having something more than "see #4642" in your code is good.