In the most recent iOS 9 beta, a new option showed up under celluar settings called Wi-Fi Assist. This will automatically allow your iPhone (and Apple Watch) use cellular data when Wi-Fi connectivity is poor.
This is going to be huge for Siri. One of the places that I typically use Siri is backing out of the driveway at home or the parking lot at work. Wi-Fi is just strong enough in both of those locations to stay connected, but have a hard time transmitting data. To a user, Siri is not working, but in reality it's just a poor Wi-Fi connection.
This feature is going to change that perception.
Update: See tweet below. While this hasn't been my experience (Siri struggles when my Wi-Fi is poor. The person tweeting does work for Apple.