While Poppy indeed covered most of it, I'm not going to waste my response:
Yes, Acritter. You can look at that 70% of doing nothing the other way around though; those are 30% of guaranteed kill. Plus, in order for a setup sweeper to become dangerous, it HAS to setup first. As said in this thread many times, two Sheer Colds in a row have 51% to hit (don't ask me about the math I have no idea how they reached it), meaning that the odds are in your favor. Moreover, Scald can burn those threats, namely Rayquaza and Groudon, crippling them and making them almost useless. Not to mention, that Kyogre's EVs where 252 HP / 252 Def with a Bold nature, meaning that it will take ATLEAST two boosts to KO it with a physical move.
I do agree that OHKO moves are quite a double-edged sword. However, it's all a question of Risk vs Reward; are you willing to take those 70% (49% if used twice in a row) that the move will miss, giving your opponent a free turn? Looking at this like that, makes OHKO moves look awful. However, those 30% leftover pretty much guarantee you a kill. So again, Risk vs Reward is the main thing that takes place when talking about OHKO clause.