The Javascript simulator allows one to pause the simulation at arbitrary times through the interactive mode. It would be useful if one could demand snapshots, and add perturbations or observables at such times.
Some models are expensive to initialize, and once unexpected behavior begins happening, the capacity to inject or observe something would be useful (instead of aborting and having to restart the simulation with a modified script). Forgetting to include the snapshot command, and figure out the right time to trigger it, is quite frustrating.