Thanks all for providing this additional context! It's great to hear how you've been using this feature. We certainly intend to bring it back in some capacity, which is where everything you've shared here becomes super helpful. 
To your most recent point, @davblo, our current work will include a review of the Preview functionality on all resource types. I think the point of the Preview button in a JS query, for example, is that it executes the JS but doesn't update the output value.