The argument is that he ruins the “brand” and that he is so unlikely to win re-election that he brings no long-term benefit.
There is an argument to be made here, but I wouldn’t call him a RINO but a liberal Republican. Bloomberg is a RINO. Scozzafava was a RINO, having endorsed the Democrat. Cao is not, although he’s an extremely unreliable vote.
I think there’s something to be said to breaking peoples’ allergy to voting Republican. If they can pull that lever for a Cao, then they might rethink why they’ve been voting Democrat all this time.
The problem with the “brand” argument is that he contributes exactly nothing to what people think of the Republicans. Its not like people say “oh, the Republicans, that’s the party of Cao”. There are no negatives whatsoever.
And the positives (breaking people’s allergies, as you say, the occasional votes, and possibly the occasional convert to something other than the non-stop propaganda heard in these districts) far outweigh the non-existent negatives.