Why is it considered a bad practice to omit curly braces?

I also used to think it's better to only use braces when really needed. But not anymore, the main reason, when you have a lot of code, it does make it more readable and you can parse over the code quicker when you have a consistent bracing style.

Another good reason for always using braces, besides someone adding a second statement to the if, is something like this could happen:

if(a)
   if(b)
     c();
else
   d();

Did you notice that the else clause is actually that of the "if(b)"? You probably did, but would you trust anyone to be familiar with this gotcha?

So, if just for consistency and because you never know what unexpected things might happen when someone else (it's always the others that are stupid) changes the code, I always put braces, because it makes the source code more readable, quicker to parse by your brain. Only for the most simple if statements, like an if where a delegation is made or is switch-like, where you know the clause will never be extended, I would leave out the braces.

If it's something small, write it like this:

if(foo()) bar();

If it's long enough to break into two lines, use braces.

Actually, the only time that's ever really bit me was when I was debugging, and commented out bar():

if(foo)
  // bar();
doSomethingElse();

Other than that, I tend to use:

if(foo) bar();

Which takes care of the above case.

EDIT Thanks for clarifying the question, I agree, we should not write code to the lowest common denominator.

Speed of reading...

Aside from what has already been mentioned. At this point, I've already been conditioned to parse if statements with braces and white space. So I read:

if (condition)
{
    DoSomething();
}

DoSomethingElse();

Slightly faster than I read:

if (condition) DoSomething();

DoSomethingElse();

I read it a little slower if it looks like this:

if (condition) DoSomething();
DoSomethingElse();

I read this significantly slower than the prior:

if (condition) 
    DoSomething();
DoSomethingElse();

beause I can't help but read it again just in-case and wonder if the author intended:

if (condition)
{
    DoSomething();
    DoSomethingElse();
}

Already covered in general, but when it comes to reading the below, I'll be looking into this for quite a while to make sure what the author intended. I may even hunt down the original author to confirm.

if (condition) 
    DoSomething();
    DoSomethingElse();