Benefits and disadvantages of writing goals--this is becoming a tough fight.
procrastination, ass-kicking and getting things done
in the right corner, and
dull routine, writing with heart and originality
in the left corner.
Kicking the muse vs Waiting till she comes
(I am oversimplifying here. Use common sense to fill the gap. If you are unable to do that, go to another answer.)
If you write every day at least 1 page/100 words/whatever, you get a lot writing done. You get a lot of practice. And you write a lot of shit.
Writing to a goal every day means, that you write when you are happy, bored, sick, have a headache, won the lottery, got divorced, buried your goldfish, shot your stepmother. Not very likely that you write brilliant prose all the time. If you are writing a novel, you will generate an awful lot of chapter rewriting--from scratch.
If you write in the right mood, if you put your heart in every sentence when you write, not thinking "I can change it later" but "it’s the best I can write", your crap-rate could be reduced dramatically. If your mind is flowing, it is possible to write almost perfect pieces of prose. Doing it while celebrating the death of your stepmother could be a little bit distracting.
But if the muse waits three months till she get her lazy butt into your writing lair, then you lack routine to listen to her sweet insinuations. The transfer of words from her lisping lips to your shivering hands is deranged--like your mind. You will despair, and while you cry in agony the dragon of failure will devour your soul.
Now go and find your own corner. Yes, you'll catch a black-eye or two.
Oh, wait, has Lauren already mentioned "whatever works for you"? No? Lazy muses ...