Short Answer: No, but the mother's needs are highly important, and it's safe to wean if she feels it's time.
At a certain point, somewhere between 6 and 12 months, breastfeeding is no longer providing everything nutritional the baby needs. From around 12 months, it's no longer nutritionally necessary to breastfeed, as the baby will be getting everything they need from real food.
This lack of complete nutrition from the breast isn't harmful in any way, as long as the baby's getting "real" food as well, and is still putting on weight, but it's important to be aware that hunger is a sign that it's no longer sufficient on its own. This is probably why the baby's seeming hungry on the breast, as her "feeding" isn't filling her up.
However, breastfeeding is also about the relationship between the mother and baby. While I appreciate you want to look at other factors, this really is the most critical thing about breast-feeding other than nutrition.
On the one hand, your baby will be drawing comfort from the feed, and both sides will be forming a bond, and this is obviously a benefit. It can also last a long time, which is why some mothers breast-feed even after two years.
On the other hand, if the mother is becoming stressed by the fact that she's not actually "filling" the baby, and if it's hurting her, then it that relationship needs to change in a controlled and supportive way, or it may start to fall apart, causing stress to all parties.
The key thing here is that, while the needs of the baby are obviously paramount, the needs of the mother are a major factor in that. If the mother, as primary carer, is under stress and miserable, that is far more important to the baby than what form of nutrition he/she's getting. (for example, calm & happy bottle-feeding mother is better than stressed & miserable breast-feeding mother)
This is the primary reason to start weaning before the baby initiates/completes it themselves.
As far as waiting for the baby to self-wean, this is obviously the best case scenario, but be aware that he/she doesn't know what she's doing. The baby won't be aware that they're not drawing food from the milk, and while they'll definitely pick up on any stress, they won't understand the reasons. They may be quite happy to wean, but just require a little push.
This babycentre link is a good source of information on how to go about encouraging the baby to wean.
Their 3 core ideas are:
- Comfort her in other ways
- Postponing feeds
- Pick your moment (Don't force it when the child is distressed/ill)