To be fair, whether that's 18 or 36 months it is difficult to imagine how being active once over that period presents a significant problem. A single partner online purchase, flight, car rental or hotel stay would solve the issue. BA Shopping and linking of bank cards do make BAEC more attractive in this case even though mostly for someone who lives in the UK.
Saying this, if I were to expect a few years without any travel at all, I would move the avios to BA for sure. Otherwise it does not matter that much.

One question I have no answer for - if an account is inactive already for 18/36 months and the avios are transferred into it - how quickly would they expire? That is, I would suggest making sure the BA account has recent activity before transferring anything.