For any chord, the most important notes are the 1, 3, and 7th (for extended chords like 9th, 11th, etc.). The 5th doesn't do much but reinforce what the other notes do, so it can be ommitted at anytime.
Also, if u have enough of those other notes, the 5th somehow sounds automatically w/out u even having to hit it. Don't ask me how, I just know it does cuz we did the experiment in class. Try this:
C major chord: C-C / E-C-E
Now, listen really carefully, and u can hear the G sounding all by itself. Pretty neat huh?