Why do we need a Reverse Proxy for Lync? TechNet (http://technet.microsoft.com/en-us/library/gg398069.aspx) specifically mentions that the following features require external access through a reverse proxy:
On Sept. 12, 2012, Microsoft announced changes to the roadmaps of several Forefront products, including the discontinuation of Forefront Threat Management Gateway (TMG) 2010. Microsoft will continue to provide maintenance and support for Forefront Threat Management Gateway (TMG) 2010 through Dec.31, 2015. This announcement of course, has spanned off 3rd parties to step in, even Microsoft with a plain Windows 2008 R2 SP1 box to address the gap.
With this announcement, there are several changes that have been taken into consideration: