I went through YC years ago so maybe it's changed, but at the time I think it was ~1 company per batch would get better terms. In other words, it wasn't negotiable. This wasn't baked in, their stance was "we don't negotiate" and only in truly extraordinary cases would they make an exception.
The reason to go through YC is, quite simply, they will increase the value of your company by significantly more than 7%. If you don't believe they can add that much value, then you shouldn't do it. There aren't many people who don't think YC can add that though, just the valuation bump you'll get while fundraising is significantly greater. And on top of that they really do a great job of actually helping you, which alone is worth the 7% in my opinion.
The reason to go through YC is, quite simply, they will increase the value of your company by significantly more than 7%. If you don't believe they can add that much value, then you shouldn't do it. There aren't many people who don't think YC can add that though, just the valuation bump you'll get while fundraising is significantly greater. And on top of that they really do a great job of actually helping you, which alone is worth the 7% in my opinion.