StanCon is for showcasing Stan. There will be lots of people there who want to try out new things. Saying, “there is a branch that does X” is essentially equivalent to “most of you cannot do X yet but will be able to at some unspecified date”. Particularly when it comes to talks, people will not be able to run the examples easily if it requires something that hasn’t been released yet.
Plus there is a lot of open PRs currently and we will probably have MPI bugs and other bugs due to 2.18 being somewhat rushed once all the MPI branches were merged.