I was always a guitar player and so I never really cared about the difference but since starting piano it is a bit annoying writing the guitar parts since I always have to transpose by an octave. Is there any reason why you can't write guitar music just the same as piano on the grand staff? I think if you use both instruments then wouldn't it make more sense to keep the guitar in its right octave?
If it was written in the same octave that it was played in, it would stray too often into the much lower parts of the grand stave. By keeping it where it is written, apart from the notes on the bottom and 5th string, most of its notes are happily placed within the treble clef, with only three ledger lines needed below that for the lowest notes.
Putting the notes into the bass clef would mean that too often, the notes played would be on even more ledger lines above that stave.
I'm following up on @Tim's answer. I didn't use a comment because I wanted to show a picture and discuss it.
As you can see, the notes that fit on a typical guitar fret-board fit nicely on the treble stave. There are roughly the same number of ledger lines above and below the stave.
I personally have trouble with more than 3 ledger lines and in the picture you will notice that this takes you neatly from bottom E on the open 6th string to top E on the twelfth fret of the 1st string. Ok there are a few higher notes but these are much more rarely played.
Guitar and piano have different sustain, different harmonicity/overtones, different purity of tone. For actual full-bodied chords, playing at the same pitch as you would with guitar chords might actually make for a rather muddy sound.
So when composing for guitar, the true pitch may actually end up a worse approximation of chordal content than playing at the octave-transposed pitch (namely playing as if the clef were not transposing). The optimum equivalent probably lies somewhere in the middle.