Should you need a third party app installed for a good reading experience? I don't use Google+ and don't want the app on iOS, even if it was launched when I click a Google+ link (which, fwiw, you are correct: it currently does not).
(On the other hand, Tumblr looks great on mobile, and it doesn't need an app. Facebook, too, is at least readable on their mobile site.)
Does anyone actually use Tumblr for original content, or for that matter, text? 99% or more of the Tumblrs I've seen are just people recirculating other Tumblr photos.
The current Facebook mobile site is almost 1:1 with their app (on Android, anyway). I wouldn't surprised to know the app is one giant WebView with a few system hooks sprinkled in.
I never understood why websites created apps of their websites. The browser is ubiquitous. Use it. I don't want to install seven different apps for my seven favourite sites. I don't see why this is acceptable on a mobile platform any more than it would be on a desktop platform.
(On the other hand, Tumblr looks great on mobile, and it doesn't need an app. Facebook, too, is at least readable on their mobile site.)