InFlames you raise an interesting point about guidelines being just that guidelines, as opposed to actual contra-indications laws for example.
Guidelines are evidence-based, but the thing is that any evidence is a collection of data, which is arrived at by taking those at either extreme as well as all those in between. There is no window on the gut, so it's impossible to tell when each individual baby is ready, it's just that babies are more likely to be ready at 6 months than before then, not that no baby is ready before 6 months.
It's also very simplistic to say that babies who are hungry before 26 weeks just need more milk feeds. My DD (15 weeks) has reflux and really struggles with bigger milk feeds. Feeding little and often is fine but she has to be kept upright and quite still (LOL!) for 30 minutes after feeding, and there also needs to be time in the day for sleep and play (not for my own amusement, but for her own development), so really, feeding little(r) and (more) often doesn't meet all her needs either. I'm not planning on weaning imminently, but I suspect by 6 months I may not be able to get enough milk to stay in her to enable her to continue to gain weight and thrive. So saying no baby needs anything apart from milk (and if hungry, more milk) until 6 months is just too arbitrary, as there are individual circumstances, some of them nutritional, some of them developmental which might need to be taken into account.
The point I'm trying to make is that, although it's a cliche, all babies really are different, and just spouting the guidelines as if they are an absolute guarantee that your baby will fit them, can to be honest be as difficult for a struggling new zero-confidence parent as the very rigid and strict routines that some books advise. It seems that routines are a little 'frowned upon' by a lot of mn-ers as we 'should' be baby-led, yet when it comes to weaning, the approach seems to be the opposite, there are fixed guidelines and no baby could possibly want or need for anything more than breast milk until 26 weeks.
Anyway, InFlames I agree, that instinct needs to play a part too. Babies don't all play by the rules, and are just as unlikely to have read the guidelines as they are to have read the book about the sleeping through the night routine etc.
As for me, when DD reaches the point where she can't keep enough milk down to thrive, then I will introduce some solids.
D