A primary key is the same thing as a unique index (any index which enforces unique entries) + a not null constraint.
It might make a difference in the heuristic for finding functional dependencies for group by though. This feature was introduced in 9.1 and allows one to not always have to list all columns in the GROUP BY clause.
I have this on my desk right now - it's been extremely helpful in scaling a large database and I'd recommend it to those who don't have any experience with indexing and partitioning as well (it explains them quite clearly). It goes into lots of system settings for tuning as well.
It's wrong, but probably unavoidable. Attaching a spoken acronym to the end of a pronounced name is just so bizarre (has anyone else ever done that?) that people will simply skip it.
I've heard people say "post-gre", and like a new English speaker saying "We goed to the park", this overgeneralization isn't at all surprising. In computers, acronyms after names are very common, and it's typically harmless to omit them, e.g., saying "Windows XP" as "Windows" is more generic but not wrong.
I've never heard anybody ever say "post-gres-que-ell". ("Postgres" is correct, unambiguous, and sounds better. Why would I keep going and make it twice as many syllables?) They're trying to fight against the natural process of linguistic clipping with a FAQ!
I love Postgres as a database but I really wish they'd just drop that weird spelling. I don't think it's helping them at all these days.