I can do a lot with QGIS (which I love), but there's no real replacements for the other tools that are cross platform. You can do a lot with SAM or pvlib, but you can't produce a bankable energy assessment using them. WINE may be a possibility, but at that point we're just on Windows.
For not work? I use a Mac most of the time, and have a laptop that runs Linux which I use for some specific tasks.
Time. Convenience.
But it's actually a really good question.