This is an interesting thread. I'll give you my take on the topic.
Preparation courses are not essential in any way. In fact, many argue that they undermine the admissions process, in that the more students are coached for specific exams, the less useful the exams become as a selection tool. But hey, as long as there are entrance exams, there will be a market for people looking for tuition/coaching/whatever, which is why the preparation courses exist.
It's important to say that the people teaching the courses have ABSOLUTELY NO "insider knowledge" that makes them qualified to teach the courses. The guys who teach at Kaplan, Blackstone tutors, Oxbridge Applications (and yes, BMAT and UKCAT Crash Courses) are just medical students (I've met a lot of them in my time at Cambridge).
When you attend these courses, what you're really getting is a short cut to preparing for the exams. For example, you can easily prepare for section 2 of BMAT by looking through CGP guides and finding the relevant areas, and then revising those topics. That is, incidentally, exactly what I did when I was preparing for BMAT, because the Kaplan course wasn't helpful at all and I needed to ensure I knew all the Science they were likely to ask. If you attend the BMAT Crash Course, you get a booklet which basically has revision notes for all 3 sciences in a coherent format, in one place. Is it necessary? No. Is it helpful? Yes.
In that sense, preparation courses are useful because the pre-work, so to speak, has already been done for you. You don't need to spent time finding the resources, you just need to practice. Which brings me to my second point - preparation courses are totally pointless if you don't put the practice in yourself. With BMAT, you might pick up a few extra points in Section 2 because a course taught you some physics that you didn't know before, but with BMAT section 1 and all of UKCAT, there's no way you're going to remember everything you were taught on a one/two-day course without reinforcing it with practice.
A lot of the time, students think of these preparation courses as magic bullets that will instantly get them a decent grade. That is absolutely not the case. If you do attend a course, you need to put just as much practice time into the exams as those who didn't attend - the only benefit is that you've been taught short cuts and techniques for answering some of the questions, and have the resources you need all in one place.
What else do you get if you attend a preparation course? Well, you get taught by people who did well on the exams and they teach you the techniques they found helpful. Did they make these techniques up themselves? Probably not, they just learnt them from a book or the internet, which means you can learn those same techniques yourself without attending the course. Attending the course just makes it easier, because again, the work has already been done for you, you just need to apply it.
At the end of the day, you need to decide if you feel spending money on a preparation course is worth it. I attended Kaplan's BMAT course, which cost £330 and got absolutely nothing out of it (except the 5 papers, which admittedly, were quite useful). I've heard though, that their UKCAT course is much better and actually does help (though it still costs £330, which is an obscene amount of money). The BMAT and UKCAT Crash Courses were created because there was no "lower end" of the price bracket - The £109 that we charge for our courses is still a lot of money, but it's far, far cheaper than the others out there, and judging by the feedback we had from our students last year, our courses are just as good (if not better) than the more expensive ones.
This has been longer than I thought it would, but just to sum up: preparation courses are not necessary, but they are helpful. It's up to you to decide whether that £109 (or £330) is worth it for the materials and the teaching that you get.