Personally, I hope folks feel free to ask other owners for their impressions and experiences, even if the manual is in the glovebox.
Most of the questions I've seen on this forum are not available in the owners manual - questions about the relative difficulty of getting Sync to understand you, the feel of the manual/automatic, and then questions for how to connect and play specific devices inside the car, whether you can get satellite without Sync and so on.
I've read the manual cover to cover (and I've read some chapters twice). The manual is written from the rigid perspective of everything operating perfectly and the user knows exactly which feature to use in any given situation and only needs to be told how to activate a feature. On the forum, I've seen lots of examples where forumites help a user figure out what's going wrong and whether something is a "bug" or a "feature" - something the manual is not particularly helpful with.
I'm not sure if anyone needs to do anything differently, but if your point was just to suggest each owner read the manual, then I think that's a good piece of advice. But posting questions about the Fiesta to the forum seems like a perfectly excellent use of the forum to me, even if the manual already covers an issue with a sentence or two.