What about throwing if at least one place is defined?
The behavior would be:

• if you don't define any place => auto-guessed from transitions
• if you define at least one place => require them all to be listed (allowing some sort of strict mode definitions to be typo-safe)

The issues comes from the framework bundle. I want people to be able to

• not define dumb places
• define places with some metadata

framework:
    workflows:
        blog_publishing:
            metadata:
                title: 'Blog Publishing Workflow'
            places:
                draft:
                    metadata:
                        max_num_of_words: 500

            transitions:
                to_review:
                    from: draft
                    to:   review

(fixed fabbot, phpunit failures seems not related)

Thank you @lyrixx.