There are probably several parameters for software quality that I’m missing. However, the point is that one should evaluate the general quality of the software based on many parameters and not exclusively “security” or “speed” or whatever.
For example, many proponents of BSD operating systems claim that the various BSDs are superior to Linux because they are more secure, or because they are (supposedly) faster or are easier to manage, because their licence is less problematic than the GPL, etc. However, they forget that Linux has some advantages like being more popular (and so one can get support more easily), or like the fact that its kernel supports much more hardware, or that it has better vendor acceptance, and because more software is guaranteed to run with less problems on Linux than on the BSDs. [linux-bsd-soft]
I’m not saying the BSDs are completely inferior to Linux, just that Linux still has some cultural and technical advantages. Quality in software is not a linear metric, because it is affected by many parameters. If you’re a software developer, you should aim to get as many of the parameters I mentioned right.
[linux-bsd-soft] Naturally, this is a problem with the fact that most developers are developing on Linux (mostly x86), don’t test it on other Unix flavours, and are too careless or unaware to write their programs portably enough.
However, it’s still a quality parameter, because it still affects the way you’re using the operating system.