NPCs, hirelings, and henchmen.
I think there is a role for these, especially in an 1e game.
The problem is you want to avoid getting in a situation where the fight is NPC good guys vs. NPC bad guys with the PCs being bystanders.
So for example, if none of the PCs can pick locks or detect traps, I would avoid sending the PCs into a dungeon full of locks and traps and have an NPC do all of the work.
I once had a party that consisted of a druid and two rogues. Lots of skills, but the heaviest armor was leather. It meant I had to be careful of combats. And first level druid spells are feast or famine, great for ourdoors, but not very useful in a dungeon.
My recommendation is to keep the number of good NPCs small, maybe one (or at most two NPCs) accompany the party into a dungeon at a time.
My personal favorite solution to the problem of having fewer PCs is to let the PCs be slightly higher (+1 or +2 levels) than the dungeon recommends. Is this a perfect solution? No. But it is relatively straight forward. And it keeps the focus off NPCs and on the PCs. If the PCs are existing characters, than the extra exp for each fight might also level them faster so they'll gain additional levels even if they aren't higher in the beginning.
The more complicated issue is the issue of party balance. The party should have rogue/arcane/divine capabilitiies. Having melee capabilities is also helpful. If you only have 3 or 4 characters, you may be missing one or more of the important capabilities (such as my druid + 2 rogues example above).
The other good part of hirelings and henchmen is you can demonstrate cool monster abilities through the classic "He's dead, Jim" approach.