I don't think using QgisApp is a good idea in any of the map tools.  If you are writing a standalone app this can't work.  I know this tool is in `app` but at some stage it would be nice to port it to gui and having dependencies on `app` makes this harder.  

Having said that.  I think all `QgsMapTool`s should have a signal called `displayMessage(QString, int)`.  This can be connected to from the main application to show a message from the tool.  Having this means if we want we can also display the message in a status bar, or a log, all from one place.

tl;dr: It's not the tools job to display the message, only that it needs something displayed.