When I did an essay on this (I finished my A levels last year) the strengths I used were:
One strength of the learning theory of attachments is that there is supporting evidence. Dollard and Miller estimate that in a child’s first year, the child is fed around 2000 times, mostly by their primary caregiver, which creates ample opportunity for the carer to form a close bond with the infant. This gives support to the idea that attachments are learned and that food plays an important role in the development of a strong bond.
Another strength is that it has resulted in the development of a newer learning theory explanation. The social learning theory is based on the idea that social behaviour is acquired as a result of modelling and imitating behaviour. Parents teach children to love them by modelling attachment behaviour. This supports the idea that the formation of attachments is based on nurture and suggests that attachments are learned through vicarious reinforcement