I'd almost say 'used to be' - most people don't think it's OK nowadays to give a baby/toddler what is essentially a hard lump of sugar that they suck for hours on end.
Source: I'm a parent of young kids, and know dozens of others as a result; I don't know a single person that gave their kid a rusk.
This made me think that there are a million special purposes biscuits. One biscuit for teething children, one biscuit for stomach ache, a biscuit for graduation, and a biscuit for Lake District walks.
Well digestive biscuits are supposed to aid digestion.
Kendal mint cake is for Lake district walks, so you don't need a biscuit.
You do raise a good point about graduation though. I assume it would have to be non crumbly so you don't lose the deposit on your gown, and be able to soak up lots of alcohol.
As biscuits and tea are such an important pairing, you won't be surprised to learn that having a million special purpose biscuits, we have done it with tea, too.
English/Scottish/Irish breakfast and afternoon, innumerable single source and black tea blends, and then there's the mass market blends. PG is the number one selling UK mass market tea but was originally sold as "Digestive tea", then "Pre-Gestee", which was abbreviated by stores to PG, which stuck.
Teabags, of course, are the spawn of satan and should be avoided whenever possible. :)
"Rusks", if anyone is curious. You dunk them into your tea or coffee to soften them before eating.