It usually depends on what's coming next, or if I want to do certain embellishments to the chord.

For example, if I know I'm going to play a G major chord after my open Em, I might use my index finger and my middle finger for the Em. That way I don't have to move my index finger when I go to G - it stays on the 2nd fret A string.

But, if I was going for a dark, evil sound, adding the 1st fret of the B string to my Em, which is a C note, would require me to grab the Em with my middle finger and my ring finger.

So yes, having multiple options is very useful indeed.