You can always use multiple sets that you roll, without declaring or taking a penalty for multiple actions, with the caveats that:
a) You can only use them for the skill that you rolled
b) you can't use multiple sets against the same target.

So the advantage of declaring multiple actions and taking the penalty is that you can use two skills at once, or attack/use a skill on the same target multiple times.

This is, of course, meant for high action games.