What are some pros and cons of attempting to forecast earthquakes/volcanoes?

Well first, cons

1.) We can't forecast the timing, so that's a biggie. The prospects of forecasting the timing for earthquakes is quite dim at the moment and not in the foreseable future.

2.) Unless you predict earthquakes 100% of the time then improper forecasting can actually result in fatalities and lifeloss.

3.) If we could predict earthquakes, there would be some huge societal implications. For instance Hurricane Katrina was predicted several days out but that didn't result in a wholly successful evacuation of New Orleans which was biased against the poor.

4.) Accurate forecasting would not change the need for preparedness and good building codes. The ground is still going to shake, buildings still need to resist lateral seismic forces, people still need to be prepared for not having gas, water, telecom, electricity, for a certain amount of time.

And pros,

It would relieve some peoples anxiety about "when the big one" is going to hit. But then that would turn into "how big is the big one" anxiety.

The questions you ask can be considered similar to "what are the pros and cons of forecasting when each of us will die". It doesn't change the fact, just what and how we worry about it.