Moodle makes a distinction between the stable version and the development version. The stable version is supposed to not receive any new features but only security fixes and bug fixes. All new features should go into the development version.
This distinction is made so that the Moodle administrator can frequently update the stable version to include the latest security fixes without running the risk of also introducing new features at the same time that might confuse the users at that institution. Even if it is a small change like having an extra question type, it could generate a lot of support requests from teachers asking what this new question type is about.
And then there is of course the issue that every new feature carries with it the potential for new bugs and having a new feature in the development version allows it to be tested on test servers before it goes onto production servers. For the majority of Moodle users it is more important to have a bug-free environment than to have the latest new feature as soon as possible.
Having said all that I must admit that we run a lot of Moodle 1.6 code on our production
server. But that is because I have conciously chosen to do that. Any admin is free to do the same.
