I don't think the main function of 兮兮 is emphasizing or intensifying.
Compare (他有点神经 < 他很神经) vs 他神经兮兮的, it's hard to locate 他神经兮兮的. Mostly, it's stronger that 他有点神经, but it can be more or the same degree. But surely, 神经兮兮的 is the most vivid (最生动形象) and least stiff (最不生硬).
I don't understand why but we tend to associate images with 兮兮. For example, when hearing 神经兮兮, a person who speak or behave that way immediately comes into my mind. The same goes with 脏兮兮, 可怜兮兮 and all the other. That's why it expresses 表示一种情态. It modifies the adjective.
It's unnatural to adjectivally use 脏 or 可怜 or 神经, etc. on their own. For example, 衣服脏，这条狗可怜，他神经 all sounds awkward (probably because this usage confuses with intransitive verbs). We usually add some adverbs of degree, like 衣服很脏，这条狗好可怜啊，他有点神经。Or we add 兮兮, as in 衣服脏兮兮的，这条狗可怜兮兮的，他神经兮兮的. Or in some cases where the adjective indeed has intransitive verb usage, we add 了, like 衣服脏了.
More with 可怜兮兮： because we see that in our mind, it brings out more empathy (惹人怜爱).