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):
You might ask how this data is created. Chitika is an ad network that shows ads on a number of websites around the world. Due to the popularity of the ad service, millions of ads are displayed daily and with that data, Chitika is able to tap in to it to see what devices are running what OS. You'll notice that the graph starts the day before where you can see less than a 1% adoption rate. This low part of the graph will represent developers running iOS 6 which has been available in beta form for a few months now. You can then see the exact point at which iOS 6 is made available and how quick the graph rises.
If you are interested in a weekly update on adoption rate then David Smith, an iOS developer, regularly updates a graph showing what percentage of users of one of his popular apps have adopted what versions. At the moment his numbers show that 13.2% have updated. This was updated on September 20th, so seems to sit about right with Chitikas numbers (this was probably captured an hour before their data).