I think that, and this is the impression I got from skimming that thread, is that raw string literals are:
1. it's only really used for regex
2. it would be better to have a function that adds escapes to regexp strings, rather than make it easier to add them.
3. it would require more work than just updating the reader.
This reasoning is solid but I think that it still falls short of justification of avoidance.
I can think of a lot of common cases which show number one isn't correct. I think the best case would be for doc strings. It would be a lot nicer to write \[func] each instead of \\[func].
Number two I disagree with on the grounds that I just don't think it's the case. For example, I do not think escaped parens appear significantly more than non-espaced parens, especially when it comes to writing major modes. Additionally, because number one isn't really the case, this reasoning doesn't work out entirely either.
Number three is just hogwash, because the "it would require more work than that" argument only works if the additional work is to make the current code handle old cases that have somehow become harder to handle. And this isn't the case. Sure, there will be come cases where old code cannot handle raw strings properly. But people can just file a bug report.
-Matt