The rendering engine is, to most non-geek users, the least important part of the browser. Third party browsers can do many things differently, including a different UI (say address bar at bottom), different interactions like gestures, different privacy enhancements (like the very article you're commenting on), different management of credentials and cookies, synchronization of open tabs and bookmarks with desktop browsers, feature addons like integrated RSS. Even among geeks who care about the implementation details there can still be differences in parts other than the rendering engine, such as the network stack (Chrome on iOS uses a different TLS stack for example).