I've ran into several posts where the author used 2 systems in the same post. Personally, I use the metric/international system all the time. Other people might use other systems, namely the US system. I'm not really comfortable using it myself but I'm fine if others prefer to use it.
The problem arise when the two are mixed in alternation, it's annoying. I think the users can use any system as long as they use it consistently.
What do others think about this?