My band does this as well- anything not tuned down on the original recording we play half step down; if it was tuned down to begin with, we leave it as-is.
I play by ear, so learning a song at the original pitch, then tuning down would really mess with my ear. I bought a program called Transcribe! that allows me to alter the pitch of any file, so if I need to lower a song by 1/2 step, I can. It makes things much easier.
To keep the open strings the same, I also tune down a half step, even thought I play a 5. For me, it makes things that much simpler.