What I didn't like about this thing (from what I can remember anyway) was not the fact she was still bfing, but towards the end the older daughter (who bf'd until she was 4 I think?) got upset because they were lying to her about when the younger one was feeding, and she felt really left out. The attatchment to the younger one but almost pushing the other away seemed really odd to me. The girl also got really distressed as she was losing her ability to get milk, so in a way it didn't, imo, come accross as self-weaning, as the psychologically she didn't see ready to wean.
It would be interesting to see if there are any negative effects to bfing that length of time in a western culture, once she realises that breasts are seen as sexual (rightly or wrongly), will she feel differently about being so attatched to her mum's? Not just in terms of feeding, but the way she almost 'owned' them.
Off topic, but I did read in a trashy magazine once how a woman expressed milk for her mum when she had cancer, and they saw a big difference, but she struggled with bfing her son and then found it increasingly difficult to express, so she just fed her directly. Wasn't really sure what to make of that.
And no I wouldn't by breast milk from the supermarket for me, I probably would for Jacob though. But I don't buy cow's milk either!