I've never had the opportunity to work with a sage. You are right, most NPC's tend to be pretty lame, but it IS one other thing for a DM to track and the new DM's sometimes have a hard time with keeping from plot-holing themselves. Although some get pretty good. Personally, when I DM, I like NPC's to be worth something but not forced to lead the players along my plots. One of the games I played in, the GM introduced us to a band of performers that were really good with weapons. Albino barbarian woman, Gnome Bard, Elvish rogue, and a weird mage who seemed to be obsessed with sheep (I didn't understand him either). The did really good in combat but the GM loaded us with more suffering to compensate for the extra help. I would have been easier, maybe, if we had been on our own without tons of other people to worry about. But it was fun.
First, thanks for the first post one our forums. We are trying to spread the word and in our next issue we will have an article informing others about the new forum, so please, spread the word.
Second, my almost 30 years of DMing has been filled with pretty much all PCs doing the following with NPCs:
DM: there is a long hallway before you and a door with skull mounted on the center at the very end.
PCs: we have the NPC go down the hall and open the door
It is a hard trade off trying to find an even ground between offering the PCs a little extra help and giving them a tool to exploit. I think this is inspiring me to write a new article

I try to give PCs NPCs that fill in some of the weak points of the party but I enforce a strict, "the NPC won't do anything the party won't" attitude. Most NPCs that the DM gives us when I play a PC tend to be very lame, not wanting to attack or not wanting to do more than attack from a safe distance and run when things get too nuts.
In the G&G campaign ran by DM Paul, a Simple Life Campaign (which is what Robillard’s Tales and his campaign building articles is based in) Paul had a really bad habit of offering us NPCs that either didn’t know anything or would help in a fight. Ardick was different. He was a sage that actually knew things and actually gave us information. He didn’t know everything, but he surprised us with an adventuring streak, like the time he followed the edge of a cave with no light to find us in an abandoned city, and he even made an attempt in combat once. We were careful not abuse the rare gift of knowledge and Ardick surprised us from time to time by aiding us even when we didn’t expect it, like when he learned a foreign language he didn’t know to further the ends of the party.
Bottom line, you should balance the NPCs. However, it is a thin rope to walk.