Simply filling is a brilliant option, but only if you've already got a handle on portion control etc. A lot of people who start WW can't actually tell whether or not they're full, because they're so used to eating until they're stuffed, and obviously simply filling isn't going to work for them. But if you feel you have that sorted, you should go for it!
If someone wanted to try but was unsure, we would sometimes recommend that they do it but still weigh food to the portion sizes recommended in the book, just for the first few days to get an idea. Most people will only do some days a week of simply filling, and some regular, but if you're doing all simply filling it's a good idea to go back to weighing every now and again - it's so easy for portions to creep up!
Seems really odd to me that some leaders advise against it entirely!