As a GM, I have used NPCs to provide temporary assistance to my group of PCs in a limited role, usually to provide advice, as part of a plot device, and to shore up a weakness in the party. I try to have these NPCs be of the supportive type and not play a dominant role, they are useful to the PCs, but not indispensable. If I need to get rid of these NPCs for any reason, I will do so.
In my current Dragonlance 3.5 campaign, the current party composition is of 4 PCs (a male minotaur fighter, a male qualinesti war wizard, a male human bard, and a female avariel wizard/bard/rogue) and 2 PNPCs (Party NPCs; a female human warmage/tantrist and a male lupin ranger/mystic). These two PNPCs provide the group with extra arcane firepower and a skilled tracker/healer, and they have proven to be a good addition to the group without taking away the spot light from the PCs.
I originally had plans to retire them at the end of the current mission, but I am not to sure about that anymore. The group has taken a liking to them, and there is a lot of interesting group dynamics between the PCs and PNPCs (not surprising considering all they have gone threw together). The avariel is romantically and sexually involved with the lupin and the female human (the latter two who are best friends), the elf and the female human get along great (both being military spellcasters), the lupin (LG) and the minotaur (CG) cant stand each other and are constant “cold war” level of tension (as both are basically good people and grudgingly recognize the value and importance of the other), and the lupin and the elf are forming a friendship. These PNPCs have been accepted as full party members by the players, and I don’t think they will take too kindly to me having them leave the group without a good reason.
Yet, since they are NPCs after all (and I have taken a liking to them) and I want to avoid having them become DMPCs, I am going to ask my players at the end of the mission what I should do with them. If they want these PNPCs to stay or leave I will leave up to my players to decide and I will abide by their choice. I think that is only fair for me as a GM and for them as players.