LibreOffice is always available in two branches: a latest release with all new features and improvements (e.g. LibreOffice 3.5.1), and a previous release line (e.g. LibreOffice 3.4). Releases are made in accordance with a defined release schedule (see below).
Not all users are able to use the new releases immediately. Corporate users with hundreds to thousands of desktops need time to test and deploy new versions in their environment. Also many Linux users rely on their Linux distributor to supply them with new versions of LibreOffice - this too takes time.
For these reasons, the Document Foundation continues to provide bug and security fixes to the older release also after the release of a newer version; however, as a general rule, The Document Foundation advises all users to upgrade to the new version as soon as possible. Corporate users are advised to use professional support services to validate new releases in their environment, advise when to upgrade, and support them through the process, including familiarising users and help desk staff with the (new) features.
It is not correct to assume that versions from previous branches are 'safer' or 'more stable'. LibreOffice software improves continuously. Newer releases are updated regularly based on the latest feedback and suggestions from users, and so, offer advantages over an older release which, of course, gets less user attention and feedback.
The LibreOffice release schedule is outlined here, and the details of our release criteria can be found on this page.
The Document Foundation and the LibreOffice developers highly appreciate feedback from you, our users. This feedback is invaluable help in improving our product.