Is there any real disadvantage to barring with your third finger instead of using your third and fourth finger to play power chords
ive got a bad habbit if playing power chords by baring wid my pinky . i think its alot easier to use both fingers though, takes less strength from the hand so ur less tense
barring with the third? i usually do the 'b minor' chord position, and just move that around. i think thats a power chord, if its not im ill informed. i think your talking about do just the 2 note power chord like 3(fret number, dont know the note) on the 5th string, 5 on the 4th, but just barred on the fret number 5 on the 4th string and one finger on the 3rd fret 5th string. anyways, i gotta buzz goin on, sorry if thats explained kinda weird.
Not really... but if you're going to make it a major or minor bar chord them barring your third finger will make that really hard or impossible.
As for playing powerchord, it doesn't matter much,
except it might not be the best posture for your hand,
A disavantage could be : when you will want to learn barre chords,
the first position is gonna be hard for you,
but the second position might be easier...

If you have a tendancy to play with only 2 or 3 fingers,
you should use your pinky in power chords to correct this habbit.
Yeah, I mean, it's easy and convenient, but should be used sort of sparingly. It's a Hell of a lot easier to use the pinky+ring finger to do that if you want to move it around very much. Provides support when moving up and down. I'm still trying to get out of that habit, honestly.
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To cut the answer short;
No. Not if you intend to ONLY use powerchords...

Now the long answer is yes. It is a disadvantage for these reasons;

You want to do the diminished powerchord and raise the fifth, well it sounds fatter with the extra root so you'll need to stop barring with your 3rd finger right there already, and that's still a powerchord.

Also, doing D string Powerchords, the fingering is different. You need to smack down your pinky one fret higher than where your 3rd finger is planted.

Not to mention barred chords, which is how I am with powerchords anyway, it's a good habit to get into, you'll be muting the other strings loose, but whacking your finger down you instantly have a full bar.
*shrugs* I don't know...
I find that using one finger for each note makes it a whole lot easier to slide up and down.
I for one prefer to play 5th chords with my ring finger barred across both stings. I find it makes it easier for me to slide and move about. Hasn't negatively affected my E-shaped barre chords either.

I guess its just a matter of what works for you.