The WordPress Theme Framework
The WordPress theme framework inserts another layer between the day to day developer and WordPress. First there is WordPress, then a parent theme and then, the child theme (see diagram to the right). The parent theme is commonly developed by a specialist WordPress development house that includes seriously smart and experienced WordPress developers. This parent theme is tasked with interfacing directly with WordPress so, when a major release of WordPress occurs (happens at least once per year these days), the guys have already made sure the parent them works with the new version of WordPress.
As long as people who are developing the child themes follow the rules set out by the theme framework we can have 100% confidence that upgrading to the latest and greatest version of WordPress will not make our site fall over for an unknown (and often extremely difficult to identify) reason. Using a theme framework also significantly reduces the development time for WordPress projects. Yes, it is more difficult to develop in a child theme environment initially, however, once you know the rules and have mastered the code then you can complete a superbly functional and great looking website in a very short period of time.