In Bloodborne, I had 12 Insight and 12 Madman's Knowledge.
At first the Insight Bath Messenger had for purchase:
The bells for co-op + Pungent Blood Cocktails; and Father Gascgoine's attire, which wasn't very good.
I just defeated a boss...
the Blood-Starved Beast, which unlocked some okay attire
Which I bought, along with a couple of items, to drop me back to 4 Insight.
At 15 Insight, the difficulty increases.
So if I use up all of my Madman's Knowledge now, I'd be at approx. 19 Insight. Would that then make any new stuff available for purchase at the Insight Bath Messenger; to deplete my Insight level back down below 15?
Because I don't see a reason to keep a high Insight level; therefore I'm just stocking up on Madman's Knowledge atm.
Update: After reaching 15 Insight, I bought all of the stuff I originally didn't want, purely to get me back below 15...
Update 2: After buying some additional crap I don't need, to get the Insight back below 15; my Insight eventually got to 16 again naturally. I've decided against buying any more crap from the 2nd Bath Messenger, and trying out the game at a harder level... maybe it has some benefit? It appears nobody knows.....