I recommend https://en.m.wikipedia.org/wiki/Wire_binding
The binding machine is heavy, but lives forever, and it's possible to get a used one at hobbyist prices.
It may look OK when new, but after having travelled a dozen times in your backpack it decomposes in the worst way. Pages start making a creaky sound when turning, they get stuck in the uneven wires, and inevitably start to separate, often from the middle of the page, which is extremely annoying and difficult to fix. When you try to re-fold the wires correctly with your fingers, it becomes worse. The only advantage with respect to spiral winding is that the actual binding act is faster to perform (but it requires a heavy equipment).
Spiral binding, on the other hand, is extremely elegant. It is the most robust shape ever, no matter how you compress it between hardcover books it always performs its job. The only way to separate the pages is by tearing the paper. You can bind them yourself with a simple multi-hole punch, and I actually find it a pleasing thing to do.
By the way, I don't understand your first comment. Spiral bound paper always lies open flat directly. On the other hand, for wire binding it may not (because the papers reach the fold of the wire).
1) Much easier to study and annotate 2) They open flat easily, so no clamps required on the music stand 3) Lighter to carry around (Nobody plays a whole book at the same time) 4) You can set up multiple pages in a row if the stand is wide enough.
1) I's easier to rest my hand on the stand and annotate if there is enough thickness to support the paper, especially if the stand is a hollow wireframe. If it's a lone sheet of paper, I tend to just lay it flat against my cello and annotate there (and I must be careful not to mark the wood underneath the paper).
2) Correct. But so do good bindings, to a lesser extent. Many bindings aren't easy to lay flat, though.
3) That depends. If you play the piano, a whole collection of studies sure will be heavy. But a melodic instrument playing a focused piece you will go through? No heavier than separate sheets: you'll use them all anyway.
4) That issue is mitigated by most good editions, where the bottom right corner of the right page is a pause long enough to let you turn. I've seen many poor editions that could be laid out properly, but weren't.
3. I play the violin and most of my music didn’t come in one piece one book. Each book has many pieces.
Only concertos sometimes occupy a whole book. Even then, they also contain the piano accompaniment scores.
4. I agree, some good editions are very considerate.
Aren't the piano score separate, so you can give them to the pianist? All concerto scores I have a separate, thin, staple bound score for me, and a thicker one for the pianist. (I don't know how they manage to turn their page with the audience hardly noticing.)
Ideally, there should be a page turner sitting next to the pianist.