You could, potentially, given the inclination and money, have a desktop computer, a full-sized laptop computer, a lightweight laptop computer, a 10″ tablet, a 7″ tablet, and a smartphone.
Which seems like it’s overdoing it. When you talk about portability, you’re really talking about the tradeoff between the form-factor of the device (larger equals more useable) and the ease of having it with you when you need it (and, to a lesser extent, the ease of breaking the device out and actually using it in a particular environment). Desktops are great, after all. They’ve got enough computing power to run complex programs, giant monitors that can display everything you need, comfortable and expressive keyboards and pointing devices, even good speakers. And they’re relatively cheap! But they aren’t there for you where and when you need them to be.
If your portable device requires a briefcase or a messenger bag to take with you, it’s inherently limited. Sometimes, you just don’t want to cart your bag around with you. It’s one more thing to wrangle when you’re seated, it makes it harder for you to maneuver in crowds, it probably doesn’t go with some of your outfits, it can be hot and if you’re on your feet enough, you might find it heavy. So I rate portability by the kind of infrastructure you need to bring with you in order to cart your devices around, and I see four major categories:
1. Not portable at all. If you absolutely need to cart it around, you need big boxes or luggage. Desktops are in this category.
2. Portable with a backpack, messenger bag, or briefcase. Laptops and 10″ tablets fit into this category. The kind of luggage you need to cart these devices around is the least intrusive kind there is, but it’s still not something you’d otherwise bring with you everywhere.
3. Portable with a purse, jacket, or cargo pants. 7″ tablets fall into this category. You don’t need actual luggage to take these devices with you: they fit into big pockets or purses. But you still might find that on any given occasion, you don’t “naturally” have the ability to tote these devices.
4. Portable in pants pockets or small purses. This is as portable as it gets (well, actually, we could imagine a fifth class of truly wearable computers, but for now at least, such things remain not-very-useful. Google Glass or the next generation of smart watches may change this). Smartphones are the stand-out example of this kind of device.
The key takeaway here is that 10″ tablets fall into the same portability category as laptops, and that this presents a problem for 10″ tablets. An iPad is much smaller and lighter than my MacBook Pro, but if I want to take either with me on an excusion, I need to break out my satchel. In contrast, my Nexus 7 fits in my jacket pocket, and, at least during winter, I’m wearing my jacket anyway. As an added bonus, if I have my Nexus out, and I briefly need to use two hands, it fits in the back pocket of my jeans — it sticks out and probably can’t stay there long term, but if all I need is another hand available for thirty seconds, that works.
If you’ve already got a smartphone and a laptop computer (and I feel like it’s pretty likely you do, if you read this blog), and you also have an interest in an intermediate device, what you want is one of the smaller tablets (the iPad Mini or the Nexus 7 are, in my opinion, the best two options, though some may plump for the Kindle Fire). The 10″ tablet is much more of an odd duck, either for real completists or for those who don’t have a laptop, or who have a truly gigantic not-very-portable laptop. That the smaller tablets are less expensive than the 10″ tablets is almost unimportant: they’re more generally valuable devices for more people.