Actually I mostly agree with you abdg - in particular about "learning by doing". You don't learn about weight and balance by writing notes about it alone, so much as you do by practicing preparing weight and balance reports. Similarly with performance - do the calculations, and again, until you can't get it wrong.
Multiple choice exams are horrible educationally, which is why very few universities use them for anything beyond class tests. On the other hand as you rightly say, they are very cheap to administer!
One of the problems with the JAR teaching system is that only a portion of the material is genuinely useful in the long term, but it isn't realistic for a PPL student to work out which. So, they do want to build up a good understanding of all of the material - which is where doing an exam first can be risky. Very few exams test ALL of the material, they have to be selective. So, you might get lucky with some aspects of the material in a practice exam, but later come a cropper when you discover that the real exam covers different aspects that are less intuitive. That's why, personally, I wouldn't be driven by a practice exam too early and put the effort into learning the material referenced in the syllabus.
G