I said it in the past and i will say it again:
The biggest strength of Linux is choice! The biggest weakness of Linux is choice!
Distros using different package management systems, different desktops and different repositories as well as different GUIs are what makes Linux great because of the choices you have. It also makes it almost inaccessible for someone looking to switch OSs easily.
You make a valid point. reading it, I can't help but think about Apple and OSX. OSX is based on BSD, but technically you could install pretty much anything Linux has on it. In the end, the main difference between OSX and Linux is really simply that Apple doesn't really push the availability of opensource software as a selling point, so it's user base contently carries on with the software that Apple sells and provides.
In those regards, Linux proponents might take a cue from Apple when helping out new users, by remembering to keep their advice as centered in the gui tools that a particular distro comes with. For example, if I were to give advice to someone about installing software under Ubuntu, my first thoughts should be to tell them how to use Synaptic Package Manager, not telling them them about apt-get, dpkg, alien, compiling, and/or other stuff, regardless of my preference towards any of them. If they want to configure something, I need to try to advise them on the gui tools available before I ever attempt to describe editing configuration files. I don't really need to volunteer the other information at that time, unless they ask for it, despite my urges to "geek out" and ramble about other options.
I suppose those are just obvious thoughts that most people have contemplated on their own, but it seems relevant.