That's not true - Apple's browsers use WebKit. I wish they used chromium though, Safari is basically the new IE.

Having all browsers use the same, open source, modern and powerful rendering engine has many benefits. It makes web development MUCH easier, improves user experience because websites work the same on all browsers (apart from any proprietary stuff the browser vendors might add on top of chromium, but that's not chromium's fault), and greatly speeds up adoption of new web standards.

I don't want there to be a Chrome browser monopoly for obvious reasons but I don't see the downside of every browser using the same rendering engine as long as it's not controlled by any one entity