The benefits of having just 1 brand of phone using an OS is that the adoption rate is far quicker. Take the iPhone for example, when a new version of the software is made available it generally works on the last 3 or 4 generations of devices and can be installed at the same time. Previously this had to be done with a wire connecting up to iTunes, but since iOS 5 it has been possible to update over the air. This is why iOS 6 is now on 15% of devices within the first 24 hours of release.
Compare the graph below to that of Android OS updates, we can see that Jelly Bean is only available on 1.5% of devices after about 2 months. That is because there are so many Android devices running the operating system that are either not compatible, or awaiting carriers to modify, build and push a new update out.
Here is a graph showing the adoption rate of iOS 6 (created by Chitika):