The great thing about iOS (and OS X) is that full screen zoom is trivial to enable and has a very simple shortcut:

Settings > General > Accessibility > Zoom

I set the zoom region to full screen. The three finger double tap is a good enough shortcut.

On OS X I use the same feature with Ctrl+Scroll to zoom anywhere on the screen. You can even toggle whether pixel interpolation is used (great if you are checking artwork and want to keep the individual pixels crisp).

I rely on these features, even though I have no trouble with my eyesight. They are wonderful and work everywhere.

The OS-wide accessibiliy zooming is just pixel zooming, though. When I zoom the viewport in a browser, it re-renders the fonts and may even show more detail on the images (assuming the images were sized down to begin with). I also still have the toolbars at the top and bottom of the screen.

I agree with you on the triple-finger tap/Ctrl+Scroll options. I use them all the time to check things out, but I still want my iOS web pages to be pinch-able no matter what the website wants

That's right. A designer who uses `user-scalable: no` is consciously taking that interaction away from you. And in general the result is sub-par, web apps which limit the viewport feel cheap and tacky.

It's a useful feature — mainly for apps which render say, an "About" page using a web view but want to make it look like a regular app page.

It's a useful feature that necessitates slower response to taps. One has to pick one or the other.

If you write your css to support dynamic type (in the case of iOS); zooming becomes less valuable as an accessibility feature.

No, it isn't because my original iPad (a.k.a. worst ripoff of the decade) is tethered to iOS 5 and can barely do anything. In fact, the fancier web sites that try to "help" me are now more prone to crashing the browser entirely.

It is important for things like HTML5 games that essentially wouldn't work without this functionality.

Then they should have had a big fat "begin playing game" button for the browser, and turned it off the rest of the time. The user should still be in control.

You can easily write a bookmarklet which you can tap that would replace any viewport meta tag with a viewport tag that allowed zooming, but as has been mentioned in other thread where this topic comes up - almost invariably the mobile browser software _already has_ a convenient user override built in. Find it and toggle it.

I understand "because games" (which are already a horrible abuse of what was initially intended as a way to display hyper-text), but far too often I wander onto some news site or blog that thinks it knows what size text I find most comfortable to read. Honestly, if you're trying to present text in the best way possible, make it easy for readers to choose both font size and column width. This lets readers adjust to suit their visual acuity and reading distance. A lot of sites seem to be designed to prevent both.