I get that this happens, after all, they use hair extensions to advertise shampoo and hair dyes, and false eyelashes to advertise mascara, etc.
But what I don't get is - are women's bodies not good enough to advertise bikinis? Sure - the manufacturers want to make their product look as good as possible - I get that, even if it means, depressingly, that only model-stature women end up being used. But will their product not look its best if it's put on an actual woman? Does it need to put on a mannequin to look its best?
Or is it a cost issue? Is it cheaper to model the wares on a mannequin which can be used over and over again, or use computer-generated images which can be created instantly with the push of a button? But surely, they need to hire a model, in order to use her face and replicate her skin colour? So they won't actually be saving any money there at all. 
I do just feel that the message is really insidious - that no human woman's body is good enough to really make our product look its very best, so we're going to have to use a mannequin/computer-generated model, which we can tweak and distort into an actual 'perfect' (read: unrealistic and therefore unattainable) shape.
This isn't about model-proportions which only a teeny, tiny percentage of women can ever hope to achieve, but about computer-generated proportions, which NO-ONE can hope to achieve.