Well, normally, chain links are straight - which means they are in pure tension.
Half links are bent - thats not good. Imagine a chain link as a £10 note. Grab hold of each end a pull it - its pretty stiff. Then fold the note twice near the ends, into a sort of 'N' shape (but less extreme) Then pull it again... You'll notice this also means they stretch more easily. And when it stretches, this weakens the material. (Thats not entirely accurate as the chain is supported sideways a little, but it gives you an idea)
Half links are generally pretty poor, its just a fundamental flaw in their shape.