Since "Dark Hole" destroys your monsters as well as your opponent's, it is important to know when to use it wisely.
First, if you use "Dark Hole" when you have no monsters, then there is no drawback. Sometimes you will even want to use it when there is only 1 monster on the field (your opponent's). Nothing wrong with that.
Second, if you are obviously outmatched and your opponent will beat you next turn because he/she has 2 or 3 monsters and you only have 1, it is better to destroy them all rather than keep your 1 monster that won't save you anyway.
Third, sometimes you will have a card like "Black Pendant" on the field and you will want to send it to the Graveyard to get its effect, which inflicts 500 points of damage to your oponent when "Black Pendant" is sent from the field to the Graveyard.