yeah, that's what it's supposed to do, but as you found out, yourself, there are some bugs with BiDir... Usually, you can (and most times SHOULD) use it, but you found one of the exceptions: alpha map shadows don't work correctly
exit portals work slightly different - at least, if you make something unexpected and
SSS currently has bugs with HIGH scattering values, which will be fixed in next release
(Did I forget any known BiDir bug?)