I’m definitely in favor of this idea too. Funnily enough, I was thinking about this some more today & the only risk that I still think is there, is a user totaling up all spend from Drinking (sub-category) + all spend from Entertainment parent category & not realizing that the sub category spend is part of the Entertainment total - so essentially point 1 from Hugo’s comments on this.
But in the worst case scenario, if this was a frequent issue, I’m sure the app could pick up on what the user was doing & display an alert.
Following on from Andy’s comment -
this is what James (who works for Monzo) had to say about this today
so there’s definitely hope. On the other hand, Hugo has liked a couple of my comments where I’ve suggested that custom categories won’t be added in the foreseeable future (& likes from Hugo are pretty rare 
). But I was explaining the benefits of aggregation too so who knows…it’s all about the anticipation 