I have read that you should not use it. Why?

For example,

<script>
	var aString = 'var a = {"json" : [{"numeric": 2, "string": "blah"}]};var i = 0;';
	eval(aString);
	alert(a.json[0].numeric);
</script>

Usually there's a better, "cleaner" way to accomplish things than using an eval, but I don't believe the rhetoric eval=evil. It's a useful feature when used properly, and the example you gave is a good one where it's used conveniently.