More practically, HIV has such a high mutation rate, that it's likely very difficult to target every HIV sequence with a sequence-specific Cas9 therapy. If the Cas9 guide sequence is too generic it'll take out stuff besides HIV (stuff you need). And if the guide sequence is too specific it won't get all the viral inserts because many are degenerate. As with all things though, 95% success with viral excision via CRISPR, in conjunction with 95% success via immunotherapy [2], and 95% from standard anti-retrovirals [3], get's you pretty good 99.9999% coverage.
That's the power of convergent technologies. It's an interesting slice through a number of modern therapeutic technologies all applied to one of the most challenging of tailored foes. You see convergence of small molecule biochemistry along with immunotherapy, along gene therapy, along with cutting edge synthetic biology - all approaching the problem from different angles.
[1] www.cell.com/cell/fulltext/S0092-8674(16)31683-X
[2] https://serotiny.bio/notes/proteins/ecd4ig/
[3] https://en.wikipedia.org/wiki/Category:Antiretroviral_drugs