The lower you tune, the thicker the strings to make up the tension.
I play in D and use 11 -52. The other guitarist in my band has a 7 string with an extran high string. His lowest string is like 56 or somthing, but tuned to d. It makes tremelo picking way easier as the string moves less. So less string motion means less pick motion which means faster playing.
Gavins right, once you learn on version of a scale with all modes, you can just play anywhere, and know where to go with more ease. Plus the fact that to change key, you just shift posistion. All the patterns will be the same if you know where the notes are.
Oh, and if people always played in scales and whatnot, you would never get cool death metal. For me, music is applied theory to (essentially) chromatic patterns, using references from real scales and/or altered sclaes with diminished, augmented, etc.
for example take the note G the first harmony is A#/Bb which is a minor 3rd from G, then you have the major 3rd which is B/Cb then B#/C is the perfect 4th, and C#/Db is your diminished 5th and D is your perfect fifth. But in E minor, there is no A#/Bb but if one person is plays G 4 times and the other player plays D C B A# it will still harmonize and sound good to the ear although you have obviously went outside of E minor. So my point is, as long as the notes you are playing, fit within a context that harmonizes with what the other people are playing, it can belong to whatever scale you want, as long as you make it flow. It works best between scales with 1 or 2 note differences, but with different versions of scales (such as harmonic minor, melodic minor) you can just go crazy with ideas for harmonies.