I'm barely a novice, but here goes:

One ... What is the reason for this type of tuning?

Open strings sound great on Celtic guitar. This isn't true in swing or other "sock" styles but there is a great deal of Celtic guitar literature that uses open strings and drones effectively.

Two ... How is it accomplished?

I don't know what is meant by "open D". I use a ton of tunings for which I have no names. Could it be (bottom to top) D A D F# A D?

Three ... Can any tune be adapted to this type of tuning?

Yes, reservedly. The question is really how well does any particular tune fit a given tuning. I believe that a tune like "Banish Misfortune" is almost universally played as DADGAD. I've refingered some DADGAD tunes my preferred CGDGAD tuning.

Four ... If a musician is using alternate tuning, is it best to take two guitars to a gig, one in normal tune and one in alternate tune?

Sounds like a good idea to me. I am too poor to have more than one "fingerstyle" guitar, so I'm stuck retuning. I'd have to take five guitars anyway.

Five ... Can one still use a capo in alternate tunings?

Yes. I use a CGDGAD tuning that I capo at the second fret. It will often return the tune to its original key. I also use EADF#BE (lute) tuning that I capo at the third fret.

Six ... What are, if any, the pitfalls of using alternate tunings?

Guitar strings have memory. I use a tuner attached to my guitar, but I often find that halfway through a song my strings want to return to their original setting. As they start going sharp or flat, my concentration drifts as well. Maybe its due to my advancing years, but I need a notebook to keep my tunings straight, life was simpler.

Seven ... Any other information, with which you would like to part.

If your reading this you must have a computer. Do you have a notation program like Encore or Sibelius (or Finale) that can do TAB? It's nifty to be able to input a melody with a simple bass line, then TAB out several tunings and look for a good fit.

Have Fun

Lucius