I’d like to note that it is in fact very common to have multiple licenses, and it does not seem like it has caused any substantial confusion, we should have noticed by now if it did.
Apart from the fact that the CC FAQ mentions it, Wikipedia has also worked a lot with dual licenses, mostly for rather strong historical reasons.
The entire ecosystem around Perl, with tens of thousands of modules has had dual licenses for decades. Qt has had several since just after the dawn of ages. There are more examples on Wikipedia.
OK, I can see I made this look too much as a Wikiworld compatibility issue. It is not, it is really more about how to encourage the growth of commons. In general, it is not only OK, but critical to the growth of certain artifacts that you can make a living around it. That enables people to put a bigger part of their time into something, and so, I simply don’t want to license stuff NC unless I think that only I should make that profit. Compiling collections, adding analysis to things for profit are amongst the things I think should be encouraged. That is something that can be done with SA but not with NC. SA then encourages the growth of the entire ecosystem in a way that not having the SA constraint does not.
More liberal licenses are appropriate if you want to achieve other effects than grow the commons, but that’s not the issue here.