I can't come up with any reason not to use them, except maybe "less to type" or "less lines of code". Both reasons equally lame.
How much of your programming hours are spent typing? And whats the purpose to strive for less lines of code? The purpose must be readability, and I would not agree that it is increased by not using curly brackets. Rather the opposite. If your readability problem is caused by functions having many lines it is the design that is the problem, not the brackets.
Instead, not using curly brackets is very error prone.
Consider that you want to add something to the code block of an if-statement, now you'll need to add the curly brackets anyway. And if you fail to notice this it will cause weird errors in your program since the second line will always be executed.
if( foo )
foo = false;
bar = true;
Or lets say that you temporarily want to remove the executed statement with a comment. Now you'll need to add the brackets, or comment the if-statement too. If you don't do any of this the if-statement will instead control the next line of code.
if( foo )
//foo = false;
As you see from these examples you may very well be fooled by the indentation of the code.
So, to all of you out there that aren't using those curly brackets unless you're forced too - start using them always!