No. Off-topic questions aren't necessary for a healthy community.
As Robert notes, you don't want to just throw your hands up and allow any topic provided it comes from someone working with a RaspberryPi device. You cannot reasonably hope to cover any possible questions of interest to Raspberry Pi USERS.
But I think the various answers to Raspberry Pi itself vs. specific OS issues illustrate that for a great number of people getting into RaspberryPi, these aren't off-topic: working with their new device is the first exposure to Linux they've ever had, and more to the point, they need to learn this stuff specifically to work with the RasPi!
But when the answer to a question can be seen as a prerequisite for using the Pi, I think that's a very good argument for allowing the question. Cade Roux hits on what is probably the best rationale for this:
They may not be familiar with the wide variety of technology which has been used to make a Raspberry Pi and adequately judge which questions should be asked here vs. Super User vs. Linux vs. Stack Overflow
Remember, the current description for the target audience here is "users and developers of hardware and software for Raspberry Pi". Not programmers, not computer enthusiasts, not *nix users, and not electrical engineers... Although you personally may fit into one or more of these groups, and this site should certainly draw on the expertise found elsewhere when possible, the purpose of creating RaspberryPi.SE isn't to fill in some gap left by the intersection of others; rather it is to carve out its own scope, one fitting the needs of its own audience.
If that happens to include a bunch of topics that aren't strictly unique to the Pi, then so be it. The task at hand is to define the scope, not preemptively prune it back.
Now, on to the meat of your proposal here. I very much like the direction you're taking with How do I install new software? - this is a great example of a question that isn't at all specific to RaspberryPi but will likely be shared by quite a few beginners. Rather than turning them away, or duplicating in-depth documentation for the various package managers, this wiki takes a pragmatic approach: a short reference, obviously intended for beginners, with links to more detailed information elsewhere.
The goal here is collaboration - which is also the reason for the continued existence of the Community Wiki feature. Rather than looking at them as "protected" exceptions to some normally-enforced rule, view them as proactive measures to address beginner questions that might otherwise prove to be a distraction.
What becomes canon?
The last piece of this puzzle is the question of which topics deserve this treatment. In general, these should emerge organically: if you're seeing new users repeatedly asking very similar questions, taking the time to write a comprehensive answer can go a long way toward easing the burden. You can edit the question to generalize it to a degree, but other than that these shouldn't normally need any special treatment.
When the goal is specifically (as with Andrew's question) to construct a collaboratively-written reference for some topic, then you can signal this with a "community wiki" post. Note that there's rarely a need for making the question community-wiki (which requires moderator intervention anyway) - anyone can post a CW answer if their goal is to explicitly invite others to jump in and contribute.
One final note on CW: avoid creating "stubs" that you expect someone else to fill in - these are just noise. And if you're not starting with an actual answer to an actual question, then you probably need to stop and ask yourself if this is something anyone actually needs.
In closing: don't get too distracted by FAQs and references
I'm glad to see that you're all thinking about this stuff already, but don't spend too much time on it up front. The meat & potatoes of any Stack Exchange site are ordinary questions and answers; "canonical" answers to generalized questions, collaborative wiki answers, and extensive tag wikis are all great tools to have, but under normal circumstances they'll constitute a tiny fraction of the site's total content. Create them as-needed, but don't let them get in the way.