Can someone explain to me why many DVDs
includes a DD 2.0 track when there's already a DD 5.1 track
on the disc?

Doesn't a 2.0 track and a 5.1 track converted to 2 channel sound
the same?

Actually they don't. A 5.1 mix will have the bass separated out to go straight to the .1 channel, the sub woofer. Many 2.0 systems, will often just throw away this bass when downconverting, so you lose low frequencies. Studios compensate for this by mixing some of the bass in a 5.1 mix into the front channels. Some 5.1 systems will properly route this back into the sub, but some won't.

So for the best playback for all, separate tracks are a good thing.

Moving to Hardware.

For those of us who don't have a fancy-schmancy setup. I always watch the 2.0 if it is included, and will even watch the mono if that is the best option for me (I have a stereo TV, but that's it... the sound I get comes from the built in speakers). Also, sometimes the producers do a bad job "updating" a soundtrack to 5.1, and people have been known to prefer 2.0 for certain movies even though they have the option of 5.1.

I remember before I realized that many dvds default to the newer track, and before I realized what a difference it made, I was watching the Terminator SE. I thought, boy this movie sounds funny on dvd, I wonder why? (I'd watched it 60+ times on VHS and knew exactly how it "should" sound. Then when it got to the first "Sarah Conn-UH?" "Yes..." "BLAM BLAM BLAM BLAM BLAM BLAM BLAM" it came out sounding like a funky laser gun from Star Wars... "Sarah Conn-UH?" "Yes..." "biyOO biyOO biyOO biyOO biyOO biyOO biyOO". At that point I figured it out and switched to the original mono track. I've read that even on a proper 5.1 system, the new 5.1 track sounds terrible.

why have those black bars when you can have the picture formatted for your TV screen? Some people prefer hearing the original audio track for the same reason most of us want OAR.

I don't think Psycho Pat is referring to older films with 5.1 remixes particularly. Columbia/TriStar and Paramount do include a 2.0 Surround track in addition to 5.1 on some of their discs, for new movies as well. As Good As It Gets is an example.

When I personally watch a DVD on a TV with no special speakers, I don't notice any difference between the 2.0 and 5.1, not surprisingly. But I'm sure people with a decent Dolby Surround system (like 5¢Deposit) appreciate it.

It seems like the studios are doing this less frequently these days, though.

Let's put it this way.......The use of 5.1 sound on a stereo only sound system, you will lose the rest of the 3.1 sound. Simple?