It may be not difficult to do lazy image loading in javascript but ideally you wouldn't have to do it at all. It doesn't encourage lazy or shoddy work. it just allows web developers to focus on creating value rather than just figuring out how to efficiently load images. It's kind of like the argument that garbage collectors are bad because encourage people to use memory inefficient designs, when in fact the reason that garbage collectors are good is because it allows you to focus on more important problems (like design and features). Sure every tool has a different way to shoot yourself in the foot but that doesn't mean you shouldn't use the tool.

tl;dr If SPDY lets me write a website without worrying about how many elements are loading at a time then ill have more time to spend on actually building a product.