Hmmm! Given that I authored books 2 and 3 of Messantia, I have a couple comments:
1. On prcs...I am not a big fan of them either. I dislike the D20 system focus on Prcs and feel that what inevitably happens is many cool character concepts get buried and lost in the play mechanics of actually trying to reach that character level. As for the abilities and use of the classes, I strongly suspect most prcs are ultimately used by GMs for NPCs, and these classes work well to help define such characters. I am not especially fond of "crunch" abilities/feats, thus the prcs have more social-focus as befitting the theme of the setting.
2. With regards to "living in Hyboria"...hell yeah, I cut my teeth on Howard and all of my gaming has been sorely influenced by his writings (I was 10 when I discovered Howard and Lovecraft back in 1980). That said, yeah, Messantia is a little more scandalous, aristocratic and, well, "conventional" than other regions of Hyboria. Part of the reason, I suspect, was that Mongoose wanted a setting that provided such a niche in Hyboria for those players/GMs who needed it. As for myself, I was working on two supplemental books to another author's work (volume 1) which had already established the theme and feel of Messantia, so I just went with it. I had a lot more control over theme and focus in my other Conan book, Tales of the Black Kingdoms, which I feel is more representative of the spirit of the setting.
3. On the social-combat rules. Actually, I know people who use and anjoy them, but I prefer role playing such dynamics myself, although again, the focus of the project called for such rule options. Even in a more aristocratic setting like Messantia, I have a hard time imagining them coming in to use very often; perhaps they would be suitable for the court intrigue of a city like Agrapur....
4. Oh, and as for monsters: Some of those appeared originally in pastiches I used as source material (Conan the Guardian was one iirc) but frankly, I just love the theme of dark and vile Lovecraftian entities as the primary source and relationship to sorcery, and such beasts appear with alarming frequency in Conan tales....my own experience running my scenario led to sheer terror among the players as the intent is to have relatively low level PCs forced to deal with these abominations that are a manifestation of the deviltry afoot within Messantia. But one should definitely tone down the monstrous encounters if they don't mesh well with your pacing of the game; if your group, say, could slam through that game in 2-3 sessions, then the presence of these monsters might seem constant. In my group, the game spread over about 10 sessions, so when a supernatural encounter erupted it was a rarer event.
Thank you all for the various comments, guys! Should I prduce another Conan book again I will take it all in to consideration.