I read (don't you love it when a post starts like that) a while back an article about locking down smartphones and not letting people revert to older versions of the OS. I wish I knew where I read it, but I vaguely remember following a link from another thread, so who knows where it was. Anyway, the Carriers are responsible for technical support of the phones and the typical lifespan for a smartphone type unit is very close to 2 years. In that two year period, it is expected that each production unit will receive (I believe the number was) an average of 5 offical updates and normally one of these is an actual upgrade to the OS as opposed to a minor update. By locking the bootloader, the vast majority of a particular model of phone, within a month of an update are running the same OS. This makes (what little bit of) support (that you might get) much easier to handle. If you have ever dealt with support for your phone and were running an older "official" OS version, the ONLY help you will get is that you need to upgrade before they can help you. Remember, with smartphones, the OS is built for the individual phone model. You cannot take the OS for the Razr and install it "as-is" on the Bionic and even get the phone to boot.
On your PC, which used to be a much more expensive piece of equipment than a phone and has a typical lifespan of 4+ years, the OS manufacturer normally handles the technical support. Yes, the equipment manufacturer in a lot of cases handles the front line of support, but will very quickly hand it over to the OS manufacturer if the problem isn't a basic setting type issue. Also, on a PC, the OS may have vendor specific crap in it, but the OS is the same as the OS on any other machine running it. In other words, if you had a Dell, a Gateway, an IBM and an HP all on the bench next to each other and one Windows installation DVD, you could intstall it on any machine you choose and it would work. Over time, even OS manufacturers discontinue support of older OS packages.
On your phone, you only get handed over to the OEM if the problem appears to be equipment related. So the Carrier supports the OS entirely. This means that you have two dozen (lets say) models of phone in thier expected lifespan in service at any time and that many OSs. If all of the phones, on average, have had 3 updates, now you have approximately 72 different OS versions possible out there. If you lock things down and force updates as much as possible, you should have only 24 OSs to support at any given time. Since your tech support staff all support multiple devices, the fewer OSs that you have at a time should mean an easier job for tech support and thus a better experience for the phone owner.
You are welcome to contrive any conclusions from the above that you like, but again it is just a replication of something that I read on-line.
Cheers,
BigRedGonzo