The way I see it, if I wanted my text constrained in a narrow column, I'd resize my browser window. Why specifically take the choice away from users by declaring in the code "512 pixels is the one and only way"? Browsers have given way too much design control over to web developers, taken from end user's choice.

As a user, I encounter three kinds of sites:

1) Sites with multi-column or table design that can show more information when the browser window is wide. I tend to widen the browser window to get the most out of these pages.

2) Sites like Daring Fireball that have a fixed width, so I can still read the content just fine whether or not the window is wide.

3) Sites that expand the line length to wrap at whatever width the browser is at. I find these impossible to read unless I narrow the browser window because I lose track of which line of text I’m reading.

Types (1) and (2) are compatible and make up the large majority of sites I visit. Type (3) isn’t compatible for me, so I find it inconvenient. It’s not the end of the world, but it slightly inhibits my desire to visit such pages. I’d rather not maintain a multi-level navigation hierarchy with multiple windows and multiple tabs in a window.

The choice hasn't been removed. You can choose not to use the author's css quite easily.

Daring Fireball, PG's blog, Hacker News and anything that is primarily text...I just click the little Reader View icon and it looks the way I want it to look.

Ecommerce sites are a minefield.

Saas and anything table-y or app-y, you're also out of luck for the most part.

This is my default for mobile - Reader view for ALL content by default - need to whitelist a few (mainly CMS-style) sites, but no ads, just text and an occasional pic.