Another advantage of writing every day is that you can take the pressure off any one writing session. If you write once a week, then that writing session has to be a very good one. You will want to write 2,000 words and make very significant progress. If you are distracted, or if something interrupts you, then, you might have to wait another week.
Writing every day, I find that some days, naturally, are more productive than others, but I don't need to put pressure on myself to make every day count. Instead, I look at those seven days of writing, which will usually add up to a signifiant amount of work.
I don't have to take weekends off from writing, because I don't need to make them intensive days. I can relax and do an hour instead of 2. No pasa nada. Right now, my writing intensive days tend to be Wed, along with Mon. and Fri. if I am not traveling. On those days I get a little more than usual done, but generally I just plug away as best I can.