seriously, there probably is benefit from doing something like this, as it activates those muscle groups only impacted a the start of what we refer to as a rep. Since they can actually lift, it makes sense to add weight.
besides, the muscle doesnt know how far its ROM is, nor where it is in the range, just how much force is applied. Usually partial reps go to the point of maximum moment...around 50% of full range in most exercises, as thats where the most power is required, but theres really no reason to go that far...you can go shorter distances but do more reps.
still, it seems counterproductive in terms of energy conservation. why waste time and effort on something seemingly inefficient?..better to make use of the time in the gym and available energy doing something more stimulating.
but hey, pro BB arent known for their grey matter, or ability to understand social and physical concepts (yet hoards of ppl buy their books and go to their seminars and ask them questions)..otherwise they'd realise the risks they are taking through their use of hormones.