The benefits are dozens of bugs fixed, some disk I/O efficiency improvements, new
features, completely revamped Web STOMP support, ability to use more recent
plugins/plugin versions.
Erlang upgrade value to me is also primarily in bug fixes, in particular when HiPE is used.
The downside of any upgrade is that software has regressions from time to time. I'd recommend
upgrading to 3.6.2 on Erlang 18.3 when it is out (we expect an RC by April 18th).
Every recent change log entry mentions how upgrades are performed. To upgrade from 3.3.x you don't
need to first upgrade to 3.4.x then 3.5.x then 3.6.x. You can go straight to 3.6.x.
New password hashing functions is another good reason to upgrade.