Ok. This is a small point so we'll say it fast.
不必 is a 副词, it can be placed in front of 动词 and 形容词.
用不着 and 犯不着 both contain verbs so after them 动词，形容词，名词都行。
譬如 = 你用不着这支笔。 You don't need (to use) that pen.
他知道是自己的错误，用不着吵架。 （你不用跟他吵架） （不必跟他吵架）
Ok. according to my dict 用不着 has two uses, 1, the standard 'no need'. 用不着这支笔。
and 2, the 'not worth it, not worth you time, pointless, useless, below your level， not worthwhile.)
And from the dict, and from what I understand, 犯不着 is used for the second definition only 用不着。
nciku also informs me that 犯不着 has a variant of 犯不上 which is exactly the same. And now I've just discovered why I have to write this entry, because I have always used used either 不必 or 不值得。