The synaptics touchpad that I am using (a newer version) has many features:
tap and drag, horiz and vert scroll bars, a scroll ball and multifinger
sensitivity -- to make two-finger tapping give the middle mouse button,
for example. 

When hw.psm.synaptics_support="1" is set in loader.conf only the scroll
ball works (buttons 4 and 5 mapped to z axis). I.e. no other features
work. Moreover, xev shows all kinds of sporatic output that seems to be
related to multifinger tapping and other features. In general, this makes
the mouse very annoying.

If you leave syn_sup off, then you get tap-and-drag but no other features.

Fix: 

It seems that with support turned 'on' the mouse is reporting more events
but they are not getting interpreted correctly.
How-To-Repeat: set hw.psm.synaptics_support="1" in loader.conf, reboot. comment that hint
out, reboot. The "support" consistently changes between the two types.