Keeping in mind that this is from a quick 30 minute review of all three products, here we go:
Pirates doesn't add any new Professions, it just gives a list of Professions from Rolemaster, MERP and Companions I, II, and III that are suitable for use although there are some rules for starting at higher Power Levels, "Leading Man", "Supporting Cast" and "Extras" with the first two categories getting some extra skills and abilities.
From a quick look, I think that "Pirates" and "Run out the Guns" cover a lot of the same material in different fashions. There is some specific equipment listed, a few skills described, some ships and ship combat rules, a section on maintaining and living on board ships. I didn't really check to see how compatible the two sets of rules are with each other.
"Pirates" does have a fairly large section describing the setting and timeline in the age of Pirates as well as several scenarios that could be played out. These sections take up about half the book and might be worth the investment to get those sections.
"At Rapier's Point" also has a list of Professions that are suitable for the setting, covering Rolemaster, Merp and Companions I,II,III,IV,VI, and VII. It also adds some new Professions, "The Diplomat", "The Noble De Robe", "The Poet/Actor", "The Religious Dissident", and "The Scientist". The additional Rules for Firearms and Grenades and Explosives look similiar to those in Pirates but with more detail. Generally there is more detail in "At Rapier's Point" which is at least partially due to the fact that it is a Rolemaster only supplement whereas "Pirates" covers Rolemaster and Fantasy Hero. As a rough estimate I'd say around 30% of the material in those two supplements overlaps and the rest is fairly unique.
Hopefully this is helpful to you.
