Why make a metro app if you can make an html5 app.
If HTML5 can't do it, metro probably can't and then a desktop program is the best solution.

MS spent lots of time trying to figure out how people use their computer, and that's fine, but they forgot that the purpose of an OS is to run programs, and developers will use whatever gives them the biggest audience and the best results (best return for time and money). And that's not metro. Metro is locked to Windows. So if you are going to develop windows only, why not make it a desktop app and get the full ability of the system? If you are trying for multi platform, solutions like HTML5 are where you are going to turn.