Why Use a Framework?
Let’s draw out the pros and cons of using WordPress theme frameworks.
Support via communities built around the WordPress theme frameworks
Typically has optimized CSS, HTML, PHP functions, and SEO
Typically has code that’s written with WordPress standards and best practices
Ease of updating for future releases of WordPress
A disadvantage of using frameworks is at the beginning, where there is always going to be a learning curve as you adopt new stuff to your development workflow. Though the learning time is relatively short compared to, for example, a new server-side scripting language, you will still have to account for this time when considering the use of WordPress theme frameworks.
However, the best way to look at using any web development framework is that it will be a long-term investment. The time you spend learning the technology at the start pays off in the end in terms of development speed and efficiency.
Another possible disadvantage is that themes built on top of frameworks, depending on the skill of the web developer, could be slower in performance than building themes from scratch, since all frameworks, not just WordPress theme frameworks, provide an additional abstraction layer.