Fire sprinkler systems trip with heat, not smoke or fumes.
True, I apologize if my inartful phrasing led anyone to think otherwise. The important part is a fire that is quickly extinguished will generate less smoke and toxic gas than one that burns longer and expands. Smoke detection has the advantage that most fires begin smoldering before bursting into flames and generating enough heat to burst the glass vial the holds a sprinkler valve closed.
To be clear, fire professionals' first priority is prevention followed by smoke alarms, and then sprinklers. Nobody would ever suggest sprinklers only.
Edit: This was written before seeing @Bob DBF's excellent post above or I would have referenced it in the paragraph above.