Red Hat doesn't support 3.x anymore, unless its 3.6 with specific subscription that extends its support.
3.4,5,6 are minor releases of 3.x.
The same way that 4.1 and 4.2 are minor releases of 4.x.
I agree that with lots of changing landscape (for example, the move from EL6 to EL7) and with the number of features introduced, they don't seem that minor. But there's an ongoing effort to both keep backwards compatibility as well continously improve quality - which regretfully, requires updating from time to time.
Or a warning sign that upgrade is not yet easy as it should be. I believe we've improved the experience and quality of the upgrade flow over time, but we can certainly do a better job.
I also think there are two additional factors :
1. Don't fix what ain't broken - it works, why bother? Not much the oVirt community can do here.
2. Newer versions do not provide enough incentive to upgrade. This is a tougher one - I believe they do, both in terms of quality as well as new features that bring value to different use-cases. However, we may not be doing enough 'marketing' work around them, or they are not documented well enough, etc.
Y.