Yes, the trinity concept is in play. On the flip side, many of the "Heroics" can be soloed (though not all) and if you call out your corner of the trinity when requesting a group for a heroic then you can avoid this issue. A word of note, make sure that when you are using the LFG tool you mark only what you can do. Sometimes the LFG tool just picks up on what you might choose not on what you did choose.