Guest wrote; "I think that NPCs that accompany the party detract from their heroics"
My friend, that kind of thinking is the first step on the road to running a munchkin campaign.
What chemical substances/ imbalance has infected your thinking Erebus?
Having a competent and or capable NPC along the party does diminish the players. It can be seen by the players as a mark against their ability to actually accomplish the scenario at hand. The reasoning being, "Oh the GM thinks we can't hack this one so he is sending help along". The other option is their thinking, "our mission is to get wizard NPC to here, so he can destroy the demon", rather than "our job is to destroy the demon". NPCs can be a slap to the players's ego.
There are times this can not be helped. You can understand why the Prince might want to rescue his stolen bride. If he is fairly equal to the PCs (or worse, better than since they might be less experienced), he becomes not a plot complication but a major force in the party. In addition, you now get to order your PCs around. (You their knight. You will ride over and flank them. You knight will stand here and use your bow. The rest of you follow me. Charge!). All very in character, all very appropriate, but all so very annoying to the PCs. Who likes getting ordered about (appropriate or not)?
The NPC, especially one which is in a "mentor" or "advisor" role, becomes an instant "get help from the GM" link. At first they will be asking for information/ facts, but eventually (IN CHARACTER EVEN), they will ask for opinions on courses of action or ideas. The NPC (IN CHARACTER) will have to respond. This puts the GM in an awkward place, especially if the NPC is wise/ intelligent/ experienced. This creates situations of miscommunication, distrust, and players guessing and secondguessing what comes out of the NPCs mouth "Do I do XXX now that he said it was a good idea. That information could be a deliberate wrong and the GM know I will do the opposite, so he told me the truth". These connundrums distract from the game and diminish the trust the PCs have for the NPCs information.
And if a compentent NPC is a long "for the ride", there will come a time where the NPC will out shine a PC. The NPC could even cut out a PC from a chance to be heroic. (Aldar(NPC) is a better rider, so he will rescue Seri from the run away horse... leaving the rest of the PCs just "sitting arround") If you try to protect the PCs and give them the heroic options, they will turn to the NPC, who will decline, and after an argument... somebody will try the heroic option... but now the PCs are wondering why this NPC is along.
This last problem, of the NPC out performing the PCs in a heroic vein, is a common GM blunder. It also leads to PCs earning less exp (divided amongst everyone involved) and having fewer options because the NPC is there. The GM, playing the NPC passibly well, will eventually take a dramatic moment from a PC. This begs the question, who's story is it anyways, the PC's or your NPCs?
Now, this is not to say you can't have minor NPCs along with the party. Send the stable boy or henchman along for support and comedy relief. Send the inexperienced boy prince along so the PCs have to protect him and not make him look bad (plot complication). However, if the NPC is competent or capable, without a debilitating hinderance (The sage is blind, the wizard is frail and old), then you have an able bodied member of the party that the PCs can not completely trust (see above) and will occasionally not perform (see above).
Munchkining, on the GM's side, most often happens when the GM has a MarySue character in the party. While most NPCs are not that bad, competent NPCs are a slippery slope that is best avoided. It is very easy for a competent NPC, run by a competent player (like say.. the GM) to be the hero of the moment, come up with the good idea, or do the dramatic. EAch time the NPC does that it diminishes the PCs, by taking away their chance.
So just keep in mind that the PCs are the hero of the saga and nothing should diminish that or take it away from them... not even the GM.