I've had a number of powerful nemesis level npcs tag along with the PCs in the past. We had a dual wielding smuggler, a lost Jedi and at one point even an assassin droid tagging along at different times. Their function was always to allow the group to go up against more powerful opponents with a safety net to help out.

Now however it's time to stop and let them fight on their own.

Anyone else had powerful NPCs tag along in the past?

In short No in long Nnnnnnnooooooooooooooooooooooooo!!!!!!!

while I understand your thinking powerful NPC can really take the spotlight from players so that they are just tagging along behind x.

First thing I would ask is why do they need to fight the more dangerous enemy? Give them ways to get around it without fighting! Or drop the difficulty, put a young one in with reduced stats. Or give them a terrain advantage.

It all boils down to your players are the stars!!!! The importance of what they do should shape the game. Not what an NPC does instead.

I'm a fan of having NPC companions for my players, but it's important to make sure that they don't take the spotlight unless the PCs put them there, or you use them as a lever to draw a certain PC into the spotlight.

Protocol Droids make for nifty companions, they can provide information or can mediate between factions to a certain extent, but are most likely useless in a violent clutch situation.

The mandalorian Bounty Hunter in my game bought a slave girl on Tatooine, and we used her to further his character. He was gonna teach her how to fight and survive and to live according to Mandalorian code.

The other time they had a passenger who just wanted a ride to Naboo, but they never got there and he was upset all the time, but he handled some of the housekeeping for the PCs and was lovingly called "Big Red".

At the end of the day it all depends on how you utilize a NPC companion. They should not provide an automatic fail safe, something like that takes away player agency.

But I think it's fine to use them to provide some quality of life to the PCs and to give them a bit of an edge in certain situations, depending on the NPCs disposition.