I think it's obvious and fair enough why BA have decided that combining is not an eligible transaction - if it were, one could merely shuffle Avios back and forth between Avios.com and BA at least once every 36 months and keep the balance alive indefinitely.

At least when you spend them, you're reducing their liability, and when you earn more, you're spending money with them (or with a partner, a tiny bit of which ends up with them to account for the Avios you earned).

Yes, that would be useful. Even Flying Blue managed to get around to implementing that very functionality last month!!!