Italics are allowed because they allow you to annotate emphasis without making the text stand out more than normal -- you only notice it inline. Allowing bold would lead to asshats writing like Jeff Atwood.
I wouldn't mind if there was a limit (maybe as few as 3) on the number of words you can make bold. Or a percentage of your post, or something. Now that I think about it the same solution could be used to support blockquote (ex: max 2 blockquotes in a post + other restrictions). After all the argument against implementing it was that excessive use of quoting is often bad style.
I think it's pretty ridiculous not to support useful features just because 1% of people might misuse it.
Italics are allowed because they allow you to annotate emphasis without making the text stand out more than normal -- you only notice it inline. Allowing bold would lead to asshats writing like Jeff Atwood.