Very true, this is why I recommended the ground school first. I try to get students to pass the written test first this way they could dedicate all their effort into flying. When you start simultaneously the flying tend to get ahead of the book knowledge, and it drags things along.
While I think this is a totally valid approach, I am not sure it is the best for everyone. Some people learn best when they have information to supplement the practical and practical to supplement the information. When I was learning how to drive stickshift, I first tried to drive and did not get it at all. Then I read about how manual transmission works (which I wouldn't have understood had I not tried to drive beforehand) and then, when I tried driving again, I was way better at it. In my opinion, I needed to kind of learn and do at the same time.
When I was learning how to fly, I did an hour of ground and then an hour of flight at the beginning. Besides for helping me learn better, I think this gave me the excitement I needed to keep going. I think I would have dropped out of the ground at a certain point had I not had the opportunity to apply the knowledge to what I considered to be the whole reason I wanted to be a pilot - FLYING! For most people who want to be pilots, the ground school is the necessary, but mainly tedious stuff you have to do to get to where you want to go. That is not to say that ground school is unimportant (it could be the difference between life and c"v death) but I don't think people get tons of excitement from the ground school especially when they are working on their primary.
Something else to consider is that once you take your written test, you only have 24 months to get your license. While this could be a "push" for people who would otherwise dilly-dally, for some people it is just a limit on the realities of their time constraints. For me, I only had summers to do my training and because of that, I almost missed the 24 month deadline (I took my written at the end of Summer #1 (with 29.9 hours), almost finished training by the end of Summer #2 (had very little time that summer) and finally got my license at the end of Summer #3 (went to my test with 47 hours)).
Finally, if you learn the ground school stuff and then take your written, it is likely that you will forget a lot of information by the time you get to your oral. Of course, a good pilot is always studying and learning, but practically speaking, the reality is that you will need to do a considerable amount of brushing up and studying before your oral if the last time you really cracked open a book was 2 years ago (or even 40 flight hours ago since your brain works on the flying and less on the book information during that time); probably quite a bit more than if you do the ground and flight together.
Again, different strokes for different folks, but those are my opinions.