Oh, lots of opinions! Read at your own peril.
> why manufacturers use "water-resistant" zippers like the YKK Aquaguard on jackets
I may sound like a broken record here, but the answer is perfectly simple: to try to gain a marketing advantage.
Please remember, the manufacturers need to sell thousands of jackets to justify the investment on that model. That means that 99% of the product will be selling to people who really have no idea about what they need or about the technology. That means they try to use marketing to convince the customer. And that means spin, spin and more spin. Spin does noty have to contain and truth or facts.
Funny - a few years ago such zips didn't exist, and yet the spin of the day assured us the jackets of that era were perfect for our needs. Think about that.
> internal stormflap vs external stormflap?
Either will work, up to a point, Neither will really keep all water out. It gets in up the sleeve, down the neck, up the skirt ... and you sweat too.
> Ultimately, I think anoraks are the right direction for rain gear
Anoraks are great for alpine use in the snow. For rain ... you sweat.
Me, I prefer a good well-designed poncho. Much warmer and less sweaty. I have worn mine in the snow and howling storms very effectively - see