Sorry for the long reply, but as this is a non-trivial and sometimes polemic topic I thought I'd share my views in more detail. If you do disagree with something, do speak up! :)
I see several advantages. They might not be particularly relevant to you, but think about it this way: there's no downside. If you want you can treat it as any other closed source platform and be on your way.
Here's what I think are the big advantages of (any) open-source project
- If you hit a particularly nasty problem you can look at and debug the OSS' code.
- If you find the cause of a bug is in the OSS, you can fix it yourself.
- You can develop a business around that OSS without being (exclusively) tied to other's goals and priorities.
- If you disagree with the way the OSS is going, or if others suddenly stop working on it, you can work on it as you wish.
- You can do whatever you want with it, even if it's something it was not part of the original plan.
Now for a bit more detail.
Most people would never do 1), but trust me - it can be handy. For every day uses most bugs and features are probably (relatively well) ironed out, but more exotic and less used parts of the system are probably not. If you venture into those you are almost always guaranteed to hit poorly or undocumented APIs, and with closed source software you'd be pretty much stuck. With OSS you just open the file and look at what's there. (You probably don't want to use undocumented APIs anyway, but the point is that if you really want or need to it's much easier to do so).
And what happens if you're developing a program and find a bug in the closed-source SDK or platform you're using? You're pretty much stuck, and have to wait until the other company fixes it. Not with OSS - if it's important or blocking, it's probably easier to just fix it yourself.
This in turn means that your business is not tied to other company's goals, that if it becomes needed you can fork the project and be on your way, which in turns means that you can do pretty much anything you want with it - whether doing it is within the scope of the original program or not.
Compare, for example, Windows Mobile with Android. (And let's not start a flamewar here - it's just an example)
Windows Mobile 7 will not be backwards compatible with previous versions, and OEMs will not be able to customize it at will. Porting it to new architectures/devices is not possible without at the very least Microsoft's approval (since they're the ones who fully own the code), and it's meant to run only on mobile phones. If you're developing a new device you're subject to Microsoft's rules and fees. If they decide to stop selling a given version, you either have to use a newer one or stop supporting the software entirely.
With Android if you disagree with a decision (like lack of backwards compatibility) you are free to either implement it yourself, or continue working on the previous version yourself. You can port it to new architectures/devices without asking for anyone's approval, and if the software is really free (and not only "you can look at the code but have to pay us to use it") there are no fees to use it as you see fit.
There are, of course, downsides. With closed-source software you can completely control the experience of users, whereas with open-source software you might fall into a trap where each device has a different experience. If many people have conflicting interests you might end up with many similar-but-a-bit-different OSS projects, duplicating effort and wasting time. The OSS license has to be chosen carefully to ensure that it's the most appropriate. If there are many interested parties you need someone who "gets" OSS to drive the project. I personally think that in practice these end up not being problems.
And what are the advantages to end users (ie the average Joe that buys an Android phone or a Linux netbook)? The first one is that it's possible to have cheaper devices, because manufacturers don't have to pay for (as many) licenses. The second is that it's easier to make new devices faster, as most of the software already exists and it only needs to be ported. The third is that it's more likely (but not guaranteed) for them to see their current device supported by future versions of the software.
In short, I can sum the advantages I see in OSS in one sentence: OSS has many advantages compared to CSS, but no real disadvantages - at worse you can use it like you would CSS, and at best you can reap the advantages.