Rumors abound that Siri is coming to the Mac. The adverb "finally" is appropriate, as Apple has four major platforms and all but OS X are Siri-enabled. Pessimist commentators have already asked, "Would you really use it on your Mac?" Of course people will use Siri on their Macs; they'll also avoid it in the contexts where they stop short of using it on their iOS devices, like in a noisy coffee shop (ineffective) or in a quiet office or library (antisocial).
"But what will it do?" seems on the face of it to be an equally dumb question, but I think it is actually relevant. With the addition of the Mac, several distinct technologies are going to be presented to users under the umbrella of Siri. Each platform — iOS, watchOS, tvOS, Mac OS (macOS?) — will have specific features that others lack. I'd only ask my Apple TV to "show me the latest episode of Jessica Jones" and it would be fruitless to ask an iOS device to manipulate files or go to sleep. But with Siri as the core brand — and it is a core Apple brand, from a marketing standpoint — core features should be present on all platforms, and the cracks of fragmentation are already forming.
A concrete example: in January, I watched a lot of Australian Open tennis on my Apple TV with the WatchESPN app. Most years, the weather during the tournament varies considerably across two weeks, from pleasant to oppressively hot. Knowing the conditions is important to understanding how the players are behaving. Siri gave me the new ability to simply ask, "What's the weather in Melbourne?" and get a response in a bottom-third overlay, as the action proceded behind it — an excellent supplement to my viewing experience. As I continued watching, I saw players' serve speeds reported in kilometers per hour. As an American, that meant I'd have to do some mental math to get an estimate in miles per hour, and really I wanted the exact figure. Siri to the rescue again, right? Nope, Siri can't help me with that simple unit conversion on the TV. On my phone, absolutely! And on the Mac…who knows? (Although my bet would be yes, since Spotlight — which, look out, could be renamed Siri — does such computations as of Yosemite.)
I don't think this is a case of "Apple is spreading itself too thin". They are spreading a liberal layer of an important technology across all their products. Siri everywhere is just one more reason for an individual or household to go all-Apple. (And yes, a dedicated piece of Siri hardware, like the Amazon Echo, perhaps should fit into that vision as well.) But if users have to constantly second guess which Siri they're using at any time, that will be a deterrent. If it's a question that Apple hasn't thought about the answer to, Siri itself won't be able to tell you either.