I do think that LOF can be a stand alone curriculum. To say that it cannot is like saying living books for science and history aren't enough. Saying that you need to have your kids do a page full of problems is like saying we can't get by on history and science without the worksheets to fill out. If you do FRED with your kids and know what they're doing, I think math can become part of your daily life just like Science and History. You need to start thinking math, talking math, etc. just like we do with the other subjects.
That said, I do have my kids do other math besides Fred because they were already doing it when Fred came along. That is why we use Fred as a supplement. If they weren't already doing the books we have, I would probably just stick with Fred alone. We have the Strayer-Upton math books and I think they would be an ideal supplement to Fred, rather than Fred being a supplement to the Strayer-Upton. KWIM? That is how I am planning to structure math with my youngest three. In fact, as I type, I am thinking about doing that with all the kids. Hmm....
And, by the way, I have not seen any of the elementary books beyond the ones we've completed, so perhaps the lessons are a little more demanding as they go on. I know that starting with the fractions book, they are definitely more demanding with more problems and tests. Except they're not called tests.