It depends what you're building. Sometimes it is useful to target one specific browser engine because there are too many discrepancies among browsers. With an atom-shell you only need to target Chromium, and then only one version. When the Chromium version changes in atom-shell you can retest everything before any of your customers see it.
Also, some of the native web views are woefully behind the times when it comes to implementing the latest HTML5 goodies (IE is notorious for it, of course, even now, but Safari is far from innocent in this respect).
