- You're not enlarging the font size, you're scaling the entire viewport. When thew viewport is larger than the screen things like modals (or other UI elements) may not appear where they are supposed to, if at all for the user
- slows down the time browsers respond to a tap by 300ms
- makes text difficult to read by the time the font is large enough because line lengths and line breaks happen off the screen now, so you're constantly panning to read your text
- users don't complete as many forms when they can scale the viewport, completing out forms is an integral step of nearly all online business models in some capacity
- without scaling disabled, a second tap within 300ms of an initial tap will zoom in on that section of the page. When developing web apps with game features I want to be sure that if the user taps too fast the page stays put
- User-scaling was not intended as a feature for mobile websites, but rather as a stop-gap to allow smartphone users to view desktop sites written by designers who had not prepared their site for mobile visitors. It's never been the plan or roadmap for mobile sites to use the zooming feature, and Apple provided developers a way to turn it off and encouraged them to do so if they add targeted support for mobile
