If students kept proper log books that covered the criteria, I'd agree with you - however, the vast majority of logbooks show all the signs of being written at the very last minute - with only a small percentage of students using them for reflection and a 'home' for valuable content. I doubt if any logbooks hardly at all are ever referred to after they have been assessed. I probably looked at a few dozen last year from schools and colleges from all over the country, and content was pretty weak. They always seem to use 'we' instead of 'I', so even determining who actually had the ideas is tricky. Actor's logbooks tend to be best, but the reality is most are written in retrospect.
The video versions, are of course, accessible - but I'd bet the number of people who ever watch them back is very, very small. The other problem with reflective work is that they often forget to say the things that they need to to get the grade. To use an example of where log books are great, we could use the TiE unit - which is very popular, and often linked with the small scale tour. One of the Distinction criteria says:
develop ideas for a TIE project that would entertain and educate with flair and imagination
A log book completed after each session is an ideal way of sorting the evidence out - the only alternative would be hours and hours of video watching it evolve, and assessing it would be horrible. So a log works fine here.
However, in the Performing Arts Business unit (one often considered a bit dull, but vital) the spec for one of the Distinctions says:
critically comment on the services provided by a range of production organisations in the performing arts analysing what they offer and how they interrelate with other areas of the industry
This could be done with a log book, or written work - but adapts nicely to talking to the camera - to get the 'D' they have to be very detailed and pick it apart - this kind of content stretches many learners because they sometimes don't have very developed writing skills - but being performers, they can talk!
Again - this is where a typical A Level student wouldn't have much of a problem producing the work via writing, but somebody who is a bit more limited with their ability to get material down in words with precision would be advantaged by not having to write it down, but just explain it.
As an ex-A Level examiner, I always found it very sad to read exam essays that were really long, yet had no red ticks in the margin, so got zero marks. Some kids just couldn't write prose. They'd have done better with bullet points, but the rules said no.