I don't like the poll, because of the limited choices. NPCs with the party should always have a story reason to exist and the players should be good with it. Some people are going to like or dislike the concept, however, no matter how implemented. IMO it's best to define some terms here, because the devil's in the details.
- DMNPC - an NPC who is equal to the party, and likely to be with the party for an extended duration. Mechanically they're usually built in the exact way the PCs are, and because of this they may outshine the PCs in certain situations. There is a serious negative connotation to these, however, as there's a long history of DMs using them as a Mary Sue for them to be a PC. If this trap is avoided, it works extremely well so long as the players (and DM) treat them no differently than another member of the party.
- Ally - an NPC who is equal to the party, but probably only for a limited duration. Mechanically they're usually just an NPC statblack, so while they might start out strong than the PCs, they will be outclassed as the PCs gain levels. These are seldom problematic, because while they may be powerful, they're also much more limited in abilities, as the statblocks are pared down from normal character creation. The DM should be careful to roleplay them based on their knowledge and personality, rather than a tool to manipulate the adventure to a desired outcome.
- Henchmen/hirelings - NPCs who are meant to be weaker and/or subservient to the PCs. They can either be NPC statblocks (short to mid-term duration) or use the new Sidekick rules (long term duration). These are almost never a problem, except for the DM who might have to do a lot during combat. This can be alleviated by delegating their use in combat to a willing player. Out of combat, they mostly just do what they're told, so unless they have a specific reason to speak up, they're often part of the background during decision-making.