I see what you’re saying. My work involves a lot of data reporting so I understand the importance of maintaining a consistent definition for a certain set of data.
Having said that, the advantage of assigning these purchases to a dedicated category is that they’re not bundled up with your personal spend. That means you don’t have to worry about the impact on targets or reporting, expenses are just excluded.
You could of course exclude anything with the expenses tag from those queries. But this is the only use case that I’m aware of for tags at the moment (sub-categories isn’t one). If they were added, Monzo would end up having to build around tags in every feature that they create. Which means we’re heading further down the path towards building a product supports business use & Tom has specifically said that Monzo won’t offer business accounts (accounts being the core Monzo product).
You’re right emoji can’t be relied on for integrations, you do need a consistent way to identify the expense transactions.