I am not one of the developers, but I believe the official response would be that SME is based on Centos, and Centos is still using samba-3.0.10-1.4E.7.
(At least, I've seen the same reason given for continuing to use PHP4 vs. PHP5).
Having said that, if there are no changes to the structure of the configuration files you may well be able to upgrade samba yourself - try it out and let us know what happens...