I used to play in altered tunings extensively and arranged a number of Scottish and Irish fiddle tunes in them (particularly DGDGCE). Now, I’ve grown to really quite dislike open tunings. Why? Mostly because it’s impractical to change tonality, or keys, in them. Play in DADGAD and your stuck in (the key of) D, or D minor, for example – going anywhere else is counter to the point of the tuning. These tunings are predisposed to playing very diatonic and modal tunes and don’t handle chromaticism well. Also, each has a very distinct sonic “flavour” to my ears – and as with any food, it quickly becomes humdrum if indulged in too frequently. In short, and for those reasons, I got very bored with them and, over a period of time, stopped using them. These days, I’ll only even go to dropped-D if it’s to play something I learned, or made up, years ago.
Different strokes for different folks, I suppose, but I like to think I can tell immediately when listening to a CD if a player’s in DADGAD. Ho-hum.
I also used to be suspicious of players using altered tunings exclusively, suspecting a lazy approach to playing since it is much easier to get something tuneful out of an altered tuning that it is in standard tuning. I’ve altered my view on this since it’s self-evident that there are players who use altered tunings all the time who can play the pants off of me.
Anyway, the reason the standard tuning of the guitar – EADGBE – is what it is is no accident; it evolved as the most efficient tuning with which to play harmonically (as distinct from modally) and as the most convenient way to play scales – and hence melodies. Or something like that…