I agree with what Gary T pointed out.

When tuned down a half step, the second string from the top is still 440-A......when fretted at the second fret.

MOST modern guitarists of the singer-songwriter type, compose around fingering, with the key only chosen by the vocal range.

Even if it is your desire to play a song exactly the same way as a given recording, tuning down just gives you one more fret before you run out of room at the top (with your capo).

All in all, the only disadvantage to this exercise of tuning down a half step is the flabbier sound of loose strings that someone mentioned earier in the thread....a problem usually easily addressed with heavier guage strings.

The only other disadvantage I can even dream of would be that you might not be maximizing the guitar's best sound capability IF (and this is a relatively rare if) you have a "tuned top" guitar. Even then, as mentioned before, just because you are tuned down a half step doesn't mean you are playing your songs in a different KEY.