barre/power chord fingering

[startingout]

Why are you supposed to use only your ring finger (on strings 2, 3, & 4) in barre chords like Bb major, while in power chords like G5(?) you are supposed to use your ring and pinky fingers individually (on strings 5 & 6)? In other words, why don't you bar the third fret in the G5 chord? If my excellent description doesn't make sense, hopefully my beautiful fretboards will help clarify...

Bb major
-------1-------|--------------|---------------|--------------|
-------1-------|--------------|-------3------|--------------|
-------1-------|--------------|-------3------|--------------|
-------1-------|--------------|-------3------|--------------|
-------1-------|--------------|---------------|--------------|
----------------|--------------|--------------|--------------|

G5 (i think its G5, anyway)
----------------|---------------|--------------|--------------|
----------------|---------------|--------------|--------------|
----------------|---------------|--------------|--------------|
----------------|---------------|-------4------|--------------|
----------------|---------------|-------3------|--------------|
-------1-------|----------------|--------------|--------------|

Thanks in advance.

[The Fury]

Hi, I think I know what you mean.

The reason you would play a G5 like that is because you play a G like this:

Example of a G major (numbers represent fingers not fets)

-------1------|---------------|---------------|--------------|
-------1------|---------------|---------------|--------------|
-------1------|--------2------|---------------|--------------|
-------1------|---------------|-------4------|--------------|
-------1------|---------------|-------3------|--------------|
-------1------|---------------|---------------|--------------|

Its also a lot easier to play minor chords because you do not have to learn a new position.

You could play it by barring the 4th and 5th string with your 3rd finger but it would be a lot more difficult to switch between chords.
Imagine you wanted to play a G5 then a G maj, you would have to switch positions if you barre the notes.
Its just easier using your 3rd and 4th fingers.