I think KamikazeMidget and ByronD in particular have both had some good responses so far. I also prefer that the numbers reflect the
semantic content of the world, rather than being wholly independent of the "skin" the DM chooses to apply.
Why is it wrong for a DM to want to say
"Ok, I want my 10th level party to fight Blackbeard's crew - here's what the AC and to hit needs to be"?
Korgoth, seriously, pre 4e, did nobody do this? Was I the only DM that did this before 4e actually gave guidelines.
I think what is said about the NPCs and monsters has to be meaningful in some way, or else I'm not interested. I don't see the descriptions as interchangeable skins... to me that's actually patronizing to the players. Like "OK now I'm just telling you this cool bit of fluff to spice up the game; I say you're fighting pirates but really it would be the same if I said you were fighting giant battlemechs or rabid fur seals."
For example, in next Monday's game of Empire of the Petal Throne (OD&D rules), the party will probably meet a barbaric warrior cult that lives in the jungle (in fact, they live in the ruined temple complex that the party just leased back in the big city). These warriors wear two things: a helmet shaped like the flaming bat creature they worship, and a loincloth. They go into battle naked to show their loyalty to the Fire Bat.
Now, their loyalty to the Fire Bat means something: it means they're AC 9.
Of course, they're "fanatics" in the literal meaning of the term (see also "fane"). So they won't run away. Their leader is pretty buff, like level 6 or 7 (not looking at my notes right now). But he's AC 9... he's a naked dude. Maybe he can cope with the party, and maybe they can cope with him. Who knows? Many of my encounters are not automatically hostile, but these boys are. I enjoy the fact that I have no idea what the party will do or how things will turn out.
As to your example of Blackbeard's pirates: it really depends on what you mean for them to be. If you mean for them to be super-dextrous and adept at parrying with the cutlass, give them some bonuses... but you should tell the players something like "These guys are uniformly agile and experts with a unique fighting style!" So let them treat the cutlass as a shield, and give them whatever bonus you would give someone for a good Dex (-1 in OD&D, making them AC 7; -3 or -4 in AD&D, making them perhaps AC 6 or AC 5). But don't just set something completely arbitrary and treat it like it's normal.
The rules and rulings (the latter more important than the former) together constitute a language that the DM is using to tell the players about the world that they're exploring. So strange and unique things can and should happen (this is fantasy!), but you don't want to put yourself in the position of talking out both sides of your mouth.
To put it another way: why not have your 1st level PCs fight a Titan? Here's a Titan for you:
Titan AC 9, Move 15', HD 1, HP 4, Att 1, Dmg 1d3.
In fact, your 1st level party could fight a bunch of Titans. They would become locally famous because they killed like 20 Titans their first time out of the village. Or Dragons. Or time-traveling German Panther tanks. Or if you want to keep things down to earth, why not use the above stat line for a bunch of black knights that attack the town? Then the PCs can slay a whole bunch of evil knights. Though I suggest using Titans because if you use black knights the PCs will want to harvest their armor.