It's always interesting to read this stuff about BF. I think we do have to be careful not to overstate the benefits though. While there are studies suggesting better immune function is linked to longer period of BFing, it doesn't mean a FF baby or a bub fully weaned onto food before 2 will be sickly.
I just thought I'd point that out because reading stuff like the above really hurt and worried me when I was giving up BF after 12 weeks. For the record, DD had 2 colds while breastfeeding and hasn't been sick once since we stopped.
To the original question - I would say that the WHO recommend BF because it's the normal and natural way to feed, which is always going to be the better option in most cases. Not all cases though.