I think it would be a good idea for monsters like Orc Shaman to gain a little bit of exp through manufacturing every time they complete something or while studying.
It should be of course significally smaller amount than through training room (so training room would still make sense). Another way would be to limit max level gained through manufacturing.
This way keeper would be able to obtain stronger monsters which cannot train in the training room or slowly level up those which are manufacturing instead of training.
