I use open chords as much as possible because they cause less strain on my hand than the barre chords. As Kevin said though, sometimes you have no choice and going from a barre to an open chord may not always be optimal for the song you are playing. I generally prefer the sound of an open chord for most things but tend to get less intonation issues (chords sound out of tune) with barre chords.
When my hand starts to hurt, I look for ways to incorporate an open chord or two into what I am playing to give my hand a rest.. just because my hand is hurting doesn't mean I can stop playing halfway through a song when I am playing live... it's not a good idea to play when your hand hurts because you can cause damage to your muscles but it's just one of those things. So if you can minimise the strain by using open chords, all the better.
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