For nearly as long as I've used a Mac, I moved my windows manually. In the days of low-res screens and perfectly rectangular window borders, this was usually a pixel-perfect art, getting several windows to peacefully coexist, tiling or stacking. More recently I realized that most of my productive tasks on the Mac had become two-window affairs. Single-window use, using full-screen mode in Lion and later, was never much of a consideration for me. It wasted space and time, as the transitions in and out were frustratingly slow on my hardware. Better to only waste time, fiddling to get the ideal side-by-side setup.
Apple also realized the utility of an in-between solution, as split screen functionality debuted in El Capitan. I was excited to try it out, thinking maybe it was exactly what I'd been looking for. One problem was that in the meantime, I'd found Moom by Many Tricks. Moom lets you precisely move and resize windows in just about any way imaginable. (My customized setup uses a handful of keyboard shortcuts. These fit into a much larger automation and key-combination scheme, which is another topic entirely.) Just a couple keystrokes can get my windows all prepped and ready to work, but they remain discrete windows. The main difference in El Cap's split view is the draggable boundary between windows.
I'd love to resize two windows simultaneously, but the other hangups of split view aren't worth it. First, getting into split view is far more troublesome than a keyboard shortcut. It's not even discoverable — I had to google to figure out that a long-click on the zoom button (a highly nonstandard gesture) was the means of invoking it. Invoking split view seems like the perfect place to leverage Siri, and even a reason to include it on the Mac. Saying "Put Safari and Mail into split view" is faster than anyone could get there with the mouse.
Then there is the question of app support, which is a far greater plague for split view on iOS, but no small problem on the Mac. TextWrangler, my text editor of choice, and it's big brother BBEdit don't do split view, for example. And resizing in and of itself is an oddity, wherein one side of the split behaves normally while the other temporarily becomes illegibly smudged and stretched. Moom's grid-based sizing is far more elegant, even if I have to repeat the action twice. As much as split view seemed designed for me, without improvement it will remain a curiosity and inconvenience just like full screen mode. In the meantime, Moom will continue to do the heavy lifting for my windows.