The main reason is that I've had to re-implement it on the server side 2 times now by sheer "luck", as well as I feel like i've just hit more of the edge cases than most just because of the areas I ended up working in (which just happened to be a handful of canvas-based apps where a couple of them needed to call out to unknown 3rd parties and I had to be very careful about using opaque responses as they would ruin the canvas).

Maybe I'm just full of myself though!