Well, I play in dropped A tuning. Why?
The informal reason: The metal band Suicide Silence is the reason I wanted to start playing guitar in the first place and I could not play their songs unless I was in the same tuning as them.
This opened up a WHOLE WORLD for me!
Remember, no matter what tuning you're in, you can always go higher, not lower. Meaning, in dropped A, I can play songs that are in ANY tuning, except for like dropped F which is sooo rare.
Any song I learn, I instantly transcribe to dropped A. I really don't see the need for me to play in any other tuning, to be honest. I play bass guitar, and higher tunings just don't have the "oomph" I need.
Look at it this way: When you down tune a guitar, you are loosening the strings. Which means the strings actually vibrate more when you play and if you pair that with distortion UGH it's amazing. The crunch is unbeatable.
Down tuning was actually founded by jazz and blues musicians. The looser strings are the trick to their play style.
And when you play say, songs that are played in standard but in dropped A, you bring your vibrato and your crunch from the lower tuning with you and create a style that is your own.
I tune my guitar like this: A/E/A/D.
So, I have a lower A and a higher A. This allows me to create melodies with the two strings that I cannot do in other tunings.
Also! The band Motograter (the band Ivan Moody was in before Five Finger Death Punch) has a horizontal bass guitar that they built that only has two strings both tuned in A. So I know when I play that band, I only need to use the 2 strings and it sounds identical! And the melodies are so awesome you would never know they only use 2 strings.