Given we have a many-to-many relation between Article and Image
When I do
article << [image1, image2, image3]
save should only be called once on the article,
and once on each image (totaling to 4 calls to #save), as opposed to:
article << image1
article << image2
article << image3
where #save would be called on both sides 3 times (totaling to 6 calls to #save)
Currently both behave the same, which isn't optimal.

I also think this is an issue when you use accepts_nested_attributes_for, so keep an eye out for that as well.