Just because its binary doesn't mean it can't be studied. Even with something like compiled native machine code its still relatively easy to disassemble and look at how it works with the right tools. With WebAssembly, dissassembly should be even easier and be more clear than native x86 or something. Still not as easy as well written javascript, but I expect it will approach the difficulty of minified js.

In my eyes, the size and offline compilation aren't even the biggest benefits. Having a low level assembly language for web computation lets people write in any language and compile it for the web. There's already an LLVM backend for it - although still experimental. But this may even mean we can simply port existing codebases to WebAssembly with little performance loss.

You've been able to compile stuff for web for a while, by transpiling to javascript, but the result has been too slow for many applications, so it never really picked up.