Taking the devil's advocate position:
The 7-zip vs. commercial is actually disingenous, because the easier route would be to just use Windows. You might counter with the Linux argument, but you've already gone out of the way to go the Linux route- no matter what else you might say about it, it's *not* as easy to install as Windows.
For the paint.net vs photoshop example- If you need the facilities of photoshop, then paint.net (as good of a program as it is) won't fit the bill. (And of course, there's that whole bit with FOSS that I don't agree with that makes Paint.NET not free software... but that's a tangent)
I do, however, get what you're saying as far as there is a happy convergence at times, which is why I said usually. However, I think that the usually still applies.