Beyond that, if there are a lot of "just barely" off-topic questions being asked, that is a good reason to launch another sub-site; this is how Super User (power computer users, any OS) and Server Fault (sysadmin) sites were launched, because there were a ton of these sorts of questions being asked in the early days of Stack Overflow, but they weren't really about programming.
We've also seen some subsections of the Stack Overflow community want their own sub-site, because they legitimately form a unique, strong community of their own: like DBA (database admins) and Mathematica. All those questions are technically on-topic for SO, but the community wants their own place, with their own reputation system, their own moderators, and their own take on what questions they'll allow.
"The danger of the dilution of the SE network into something like Quora"
This made me laugh, but Quora's problem is that they allow and even solicit pure opinion, whereas on Stack Exchange we prefer answers that can be verified in some tiny way. "Because that's what I think" is a valid reason to answer anything on Quora, which gets into unpleasant weirdnesses like judging answers based on the fame/success of the answerer, because, well, what other science can you apply to an opinion?