1. WebKit is not created equal – if you need to test anything related to system integration (e.g. font rendering, video/audio, etc.) you need real Safari. You're also exposed to differences in the WebKit version + vendor patches.

2. That doesn't help at all if you need to test against Mac or iOS clients and want the underlying layers to match. I've found a ton of Unicode-support related issues rendering text properly which worked on Windows 7 or maybe only 8 but not OS X (or earlier versions of windows, etc.). Similar problems exist for HTML5 media playback.