You're forgetting about the significant overhead imposed by every process having its own copy of the necessary libraries loaded into memory, and having to load those libraries from disk every time an app using those libraries is started. If those apps were all using a version included with the OS, all those pages could be shared between processes and only loaded once.
But maybe people don't care about apps starting fast on their macs anymore, since a lot of them come with SSDs :)
I don't believe you. Apple includes OS standard versions of packages like SQLite and Python, are you of the opinion that developers shouldn't use them?
If they are standard and public and they work, sure. If they're not public and not standard (like MacRuby) then probably not.
I think the GP post was simply referring to the fact that a vast majority of software is 'installed' on the mac. Just drag it to the application folder. And drag it to the trash to uninstall. This almost certainly means you have a bunch of duplicated library files on your computer, but also means each app doesn't have any external dependencies.
As is required to make sure that there is no data loss. Yes, it is a performance loss as well, but it makes it much better with regard to possible crash scenarios.
But maybe people don't care about apps starting fast on their macs anymore, since a lot of them come with SSDs :)