ok, this is the info I was told by three different surgeons/clinics in three different countries:
Pre-op diet is primarily for (1) shrinking liver, since it has to be kept raised during laparoscopy surgery to access your stomach, at least the upper part where they place the band (2) for very high BMI, any surgery will be less dangerous, regardless of having to lift the liver, if you can drop lots of weight. All the BMIs I've seen in this thread thus far are significantly under 40. Mine was 36 when I first started contacting clinics (but then got up to 38, and stayed there till the night before my op).
None of the three clinics I spoke with told me that a pre-op diet was strictly required in my case. One doctor told me that it would make absolutely no difference (BMI 36, mind you). Another doctor told me that it would be a good idea. The third doctor recommended a high protein diet (low fat, such as pure protein shakes) for only three days prior to surgery. Given all this information, I tried to do the pre-op diet 2 weeks prior, but completely failed, in part because I knew in my heart it wasn't a dire requirement, yet it sure was difficult to do! In the end, I only did 2.5 days of strict diet pre-op. When I asked my surgeon if my liver was larger than it should have been, he said absolutely no, there was no problem.
Oh, also, important to mention, that the pre-op diet has another purpose: to get you started, mentalized, to get the ball rolling, on a new way of eating... on a new life, particularly right after your operation that you will be on liquids for a month!