GM Question: Running the show, with a PC in tow

This is probably going to sound silly. Still...

Who knows why you'd want to, but say you're setting up a thread with one of your avatars as a party member, not just an adventure hook. How do you deal with this fairly?

The ways it can go wrong, as I see it, are if the GM-run Player Character you give them is too good, or too worthless.

If they're too good, they know the answers to problems too quickly, destroying the fun, and they probably drive the plot in the direction you want them to, all the time -- running the game on rails. I know full well that running on rails will just get people annoyed, so this is a bad plan.

If they're too worthless, they're basically just dead weight, sitting in the party's shadow. They never get involved enough in things to make a real difference, and they're not all that helpful with puzzles, so why are they along in the first place? If they're a Macguffin, it's not really a PC anymore, is it?

So, in case one of us ever feels insane enough to try doing this, how do you have such a PC that's still useful to the party, but doesn't end up running the thread on rails? How have people dealt with this in the past?

And yes, having a party member for your folks when you're GMing in tabletop, or similar, should count as well. Threads are just what's most common, here.