Never understood the Google and bing apps myself. And with spotlight in iOS 8 and Siri, I see even less use for them.
YouTube especially. The YT app is nearly the same thing as the mobile site except that it has advertisements. Safari just loads the MP4 video directly from its URL, so there are no ads, and I think there's some throttling involved in the app. I've noticed (though not with official testing) that YouTube's Flash, HTML5, and native iOS players nearly stop loading the video if you pause, and they seem to load more slowly even when you are playing than if you load the MP4 directly.
So my setup is Click2Flash in Safari on Mac and Safari on iOS to completely avoid YouTube's video player.