But it's a nice way to quickly visualize a fractional Bitcoin quantity.
If it's something like 0.01000000 you pay attention to the first digits and think in terms of BTC(0.01₿), and if it's a small number like 0.00001000 you pay attention to the last digits, and think in terms of sats(1000 sats).
Exactly. One satoshi is such a small value today that we don't really need mBTC except maybe when talking about Lightning stuff, but even then you can easily get away talking about 0.5sat if you really need (instead of 500mSat). I expect if / when sub-satoshi amounts will be relevant for pricing out stuff, we will have moved to a mostly satoshi-based standard anyway so we'll talk about something costing 1.99 sats and having paid a LN fee of 10mSat maybe. But never will we really need to denominate something in a BTC fraction with 10 or 12 decimal places.