Anyway, imho the rule should be: use plain name for the latest stable release, and add the appropriate suffix (usually 1 or 2 digits) for any other release. This is wonderfully simple and straightforward. It's also current practice. Let's keep using it.
There are basically 2 scenarios to consider: parallel branches and transition branches. Can you give an example of the "parallel branches" scenario? Even Python 2, a poster child for long-term backwards compatibility, is still a "transition branch". I can't think of any examples of the "parallel branches" scenario, and if that scenario does not exist, then this entire debate over packaging standards is a moot point.
I think a better policy would be to add a defining suffix to the main stable branch package when future incompatible branches may be expected and when the current main stable branch is clearly distinguished The main stable branch of a project is *always* going to be incompatible with future versions. Django 1.6 will be incompatible with 1.7; Lighttpd 1.4 will be incompatible with version 1.5; and so on. That's just the nature of software.