What are pros and cons of submitting a PhD thesis to ProQuest?
I don't see any advantage to posting your thesis to ProQuest, but I don't see any real disadvantage either.

Since your thesis is already on ArXiv, it's already permanently (and freely!) available. So I don't see any advantage to also making your thesis available through a more painful interface that requires your intended audience to give other people money. Presumably the results in your thesis will also be (or have already been) published in other indexed publication venues, so even having your thesis indexed by ACM isn't a clear advantage.

You can list your thesis in your dissertation even if you don't submit it to ProQuest. In fact, you must list your thesis in your CV, even if you don't submit it to ProQuest.

(JeffE's answer.)