Kaleea and the others are right, you don't often have to worry yourself about modes. My excuse is that I play Mt. Dulcimer and it's fun to tune and play in the appropriate mode - the dulcimer sounds very different depending on how it's tuned. The dorian and aeolian tunings produce a very haunting resonance very unlike anything I get on my guitar or even the dulcimer in a more orthodox (ionian or mixolydian) tuning.

Actually, the guitar player for the sessions I take my flute to rarely plays chords other than D or G. Much of the time she plays a simple accompanyment of single bass notes in a moving line that follows the implied harmony of the piece. Very effective.