One potential issue is that data prices in the EU differ quite a lot between countries. For example, I have an Austrian drei.at prepaid card where I pay around 10 EUR per month for 10 GB of data (I have to recharge the card at least once a year so that it doesn't expire). In some other European countries the price for one GB of data is more like 10 EUR instead of 1 EUR.
So one of the use-cases for perpetual roaming is to buy "cheap" SIM cards from another country and to then use them in your (more expensive) home country, which would lead to an asymmetric roaming behavior between the two providers (so one of those providers would have to pay the other one).
My example was about retail prices: You're correct that due to clearinghouse there are hardly any charges exchanged - as in most cases the outgoing roaming volume of a provider is comparable to the incoming roaming volume.
But if you have two countries where retail prices differ a lot, this can lead to people from both countries to buy SIM cards from the cheaper country, causing an asymmetry in the roaming volume: Now suddenly one provider has lots of outgoing roaming, but much less incoming roaming - which means that this provider is going to have to pay the wholesale prices for the difference in roaming volume.
So one of the use-cases for perpetual roaming is to buy "cheap" SIM cards from another country and to then use them in your (more expensive) home country, which would lead to an asymmetric roaming behavior between the two providers (so one of those providers would have to pay the other one).