score:5
Since the card is prepaid, is there any reason to not keep using it until it cuts off?
You might want to keep it for voice use only (where roaming charges are not as high) as a backup or when you enter a new country and don't have a local SIM card yet. Theoretically, you can also cash out unused credit under some circumstances, but it's probably not worth the hassle.
(And when we that be? There's β¬10 of voice credit with the account.)
It depends on the details of your Belgian provider's terms of service, but typically the roaming data usage will eat into that credit, and once it's used up (probably after a few MB) it will stop working. There are some prepaid plans that will automatically top up the credit from your bank account or credit card every time it runs out, but this is usually a feature you have to activate separately (and should, of course, avoid).