I don't think the Unix philosophy makes too much sense for things other than CLI commands, and even there, I'm not 100% convinced.
- Write programs that do one thing and do it well.
- Write programs to work together.
- Write programs to handle text streams, because that is a universal interface.
(3) definitely makes no sense outside of CLI commands, (2) is too ill-defined to be of any use (at least outside of programs, though even for programs it seems to be quite redundant with 3), which leaves (1)... which is basically a matter of taste: "one thing" is an extremely ill-defined concept, and you could easily argue that most of the useful SUS commands break it, especially (though not exclusively) if you look at it from GNU's coreutils.
I'm not sure I'd say that. In the web development world, you'll definitely see people arguing to "JSON all the things" (text) but others arguing to "protobuff all the things" (binary). And they raise many of the same simplicity-vs-performance issues that came up for Unix CLI commands.
As for (1) and (2) being too ill-defined/a matter of taste – well, I agree, but I don't think that means they're useless. I think of them as being in the same category as advice for writing prose ("Use short sentences where possible", "Avoid cliches"): helpful goals to keep in mind, even if I can't pin down exactly what they mean.
Like for example, an HTTP server or client
No computer ever, has done something whose chain of causality does not return to a matter of human choice in orchestrating the circumstances for said outcome.
You can tell it's a little religious because, like Agile and REST, "everyone is doing it wrong". Where the thing everyone is doing wrong is a weird little corner of the thing with dubious utility.
In desktop applications I usually prefer tools that cover the majority of use-cases and provide an easy way to extend them (the last part is important too, otherwise you end up with the Windows 8 default apps). It doesn't make sense to split up a photo editor's core featureset, but that doesn't mean it's a good idea to just bury it in a pile of features out of the box that only 5% of users care about.
Maybe it made sense at the time.
There's still plenty of GUI tools in the FOSS space that don't heed freedesktop conventions. There's also still people that bitch endlessly about D-Bus, systemd, any anything that's not just piped plain text everywhere.
Thunar can unzip files. Oh no it's doing two things! Shouldn't it leave that job up to `tar`? Xfce4-terminal has tabs. Erm shouldn't that be the job of the window manager like on Mac? Stop doing more than one thing!!
You could potentially do it with something like D-Bus I suppose. But it would still probably be simply better if it was built in.
D-BUS???
2. You have to do all communication through argv and parsing stdout which is extremely error-prone.
3. You're almost certainly not going to get proper progress reporting.
4. It can't be interactive (e.g. how do you implement the "This file exists, what do you want to do?" dialog?)
It's so obviously worse I'm surprised you even asked. I'm not sure what "D-BUS??" means but D-Bus mitigates a few of those problems at least.