Any practical decision is going to hinge on who you're trying to target or what you're trying to accomplish.
If you're talking about swaying the general public, convenience is a much higher consideration than the openness of the underlying source code. The ratio is approximately infinity in my experience, even amongst people aware of the difference and on board with the goals. I have no idea how one might change this, but I doubt writing any number of programs is going to do it. It's more of a political or social issue.
E.g. an OSS GUI app that would make people flock to Linux today? Even if it got enough popularity for the general public to notice somehow, it would just ported to other platforms (or independently recreated) before that happens.
If your main concern is just that more people use OSS in any form, you'll want to run on as many platforms as possible. By the numbers, you should actually target non-Linux first, in fact. (Unless you happen to be targeting one of the niche markets with a lot of Linux users, but then those people are already OSS users.) Or how about an OSS GUI platform for use in consumer electronics? More of a case to be made. The cost and convenience equation is much different for that market. But again, almost no end user is going to care how open it is.
On a technical note, the nature of the application is also a consideration. Only some sorts of programs are "worth" running as a desktop application now; others have been usurped by running a mobile app, within the application platform we still call a browser, or some combination of the two. If there's a chance your app could run in those environments, you probably do want more of an OS neutral approach.
If you're talking about more of an ethical or personal decision, that's a different conversation again.
I'm also not sure I understand what they meant by that phrase, but I wouldn't use the phrase "all inclusive" for the GPL either. To me that invokes the (almost always incorrect) "open source means I can do whatever I want with the code" interpretation that gets used so often casually. As someone who supports the sentiments and goals, I feel it's important to emphasize that there are restrictions imposed. Namely, copyleft licenses forbid using the code in non-copyleft programs. Or to rephrase, they keep code open by excluding those unwilling to open their own code too.