The harvest will be less then optimal because the tree will spend its energy producing inferior fruit.

Thank you, it makes perfect sense when you think about it!
I read somewhere that this fig is hardy down to -10. Does this mean that above that it doesn't need wrapping up in winter? Or does it need protecting from all frosts, if so is it just the roots, or the branches too?

14º F, Bathgate. Tcc, I would think about wrapping it in cold winters, and giving it a sunny southern exposure, to help with fruit ripening, especially if you live near the coast.

Thanks Tugbrethil, but presumably it still needs light through the winter? So is it a case of wrapping it at night and unwrapping again every morning in frosty weather? Sorry if that's a daft question, but I'm just not sure and I really want this tree to succeed.

Light needs are low in winter, since it drops all of its leaves. The main purpose in wrapping is to preserve the bark at the base, so that a sharp freeze doesn't girdle the tree; and to preserve some buds, so that the tree doesn't have to start again from nothing the next spring.
So, the short answer is to wrap once, and leave it wrapped until early spring.

Thanks again, that's very helpful.