I have been using Common.Logging for four years, and I'm able to use it with NLog2. To answer your question thoroughly, we need to take a closer look at Common.Logging first. Common.Logging gives you two benefits:
Since 1.x, Common.Logging let you write your application independent of the logging framework. So you can easily change from one logging framework to another without even recompiling your application. This is particular useful to the development of common library, which may be used by various applications that use different logging frameworks.
Starting 2.x, Common.Logging let you aggregate the logging information from various logging frameworks. Let's say we are developing an application and want to leverage two 3rd part libraries, say A3rd.dll and B3rd.dll. A3rd.dll uses log4net, but B3rd.dll uses NLog. Now how do you consolidate the log information from A3rd.dll and B3rd.dll into one log file (or log monitoring system)? Common.Logging can help, for example, it can capture the log message from NLog and send it over to log4net, and then let log4net to write it to the log file or send it to anywhere that log4net can do.
Now, back to NLog2. The logging API in NLog2 is backward compatible with NLog1, but the API for configuration and targeting were changed. So if all you need is to send the log message to NLog2, you can simply do assembly redirect (see my answer here: Can NLog v2 be used with Common.Logging).
That being said, if you want to aggregate NLog2 message to other logging framework using Common.Logging, the assembly redirect approach won't work. A NLog2 specific adapter has to be created.
Like @Kugel said, inactive could mean stable and mature. As Common.Logging works with various logging framework, it is unlikely to release a new version every time one of the supported logging framework makes a release. It could be otherwise more confusion than help. Hence, assembly redirect should always be considered first. Only when there is really an incompatibility issue arise, like I mentioned about the NLog2 message redirecting, send an email to the mailing list and I'm sure somebody will jump in and help.
Cheers,
Kenneth