Google has said that they've bought Motorola for patents, and some say that perhaps they changed their mind in the mean time. But I think Google wanted all along to become a hardware manufacturer that has complete control over both the software and the hardware. There are a lot of money to be made from hardware - a lot more than from software or services - if they do everything right.
Google has become better and better at specifying the technology for the Nexus devices. They are certainly not a laggard in this area, compared to say Nokia or RIM, which still can't put out cutting edge hardware on the market. So I truly believe that given the chance to actually own their own hardware company, they could make some of the best mobile devices out there.
And rumors point that that's exactly what they are intending to do with Motorola. But this raises the questions what will the other manufacturers do about it? I think they would have 3 kind attitudes towards this, two that should be ignored, and the other that could easily be addressed by Google.
The first one is that companies like Samsung or HTC would obviously prefer Motorola to die or at least not be a very strong competitor against them. All manufacturers would think that, but it's really unreasonable for them to think like that and this concern of theirs should be completely ignored. Motorola gets to be a strong competitor against? Well, tough luck. That's how business is done in the market.
The other concern would be that Google would treat Motorola much more favorably. First of all, it's probably inevitable to have some kind of favoritism between Google and Motorola. But as long as they give them access to Android in the same time as Motorola, and they don't lock them out of some features, then there's really nothing serious to worry about in relation to this.
Another possible concern is that is that Google will just give Motorola the freshly off the oven Android version to put in their devices, and sell them like that in the market with stock Android. And they would fear that Motorola's devices would be more popular. To this I say once again: tough luck. This is an option that every single manufacturer has, but time and time again they have refused to adopt it, and instead have preferred to use their own skins. If they are really that scared that Motorola's devices with stock Android will be more popular, then they should simply use stock Android, too. Problem solved.
Some suggest even an extreme scenario of manufacturers banding together to force Google to sell Motorola - or else. Or else what? Well they could say they would stop using Android. Alright, manufacturers. Good luck with that. See how many smartphones you're selling as soon as that happens. They could also say they will use another OS or invest more in WP7. Once again. They've already done that, and they've had no luck with it.
But even if this extreme scenario actually materializes and somehow only Google/Motorola are making Android phones, I'd probably still prefer it. Most people want Android phones today because they like Google and what Google is doing with Android. Google can take the entire ecosystem with them, plus they would make some killer hardware, too. Even if they wouldn't have the biggest market share out there, I'm sure they'd still do very very well in the industry with their own Android phones.
But again, this is just one imaginary crazy scenario that will never happen. Companies depend too much on Android today, and others have tried and keep trying to use other mobile OS's with little luck. Nothing has come close to the success of Android or Apple's own ecosystems.