The battle lines between tablet vendors were so clear a year ago. Heading into the holiday season, suppliers for the most part believed that consumers wanted either a 7-inch tablet or a 10-inch tablet. This year, though, they're offering one of each.

Apple and Amazon, two of the more visible champions for 10-inch and 7-inch devices, respectively, have since come to appreciate the other's perspective, as they've rounded out their 2012 holiday lineups with tablets targeting the other's wheelhouse. But why? Do they view size as a matter of taste, as it is with mobile phones and motor vehicles? Or do they think that the same consumers want both?

I'm here to tell you that before long, you'll want one of each -- even if you don't right now. And tablet makers who don't understand that emerging dynamic today will figure it out soon enough.

Now wait a minute, you may be saying. Aren't you the guy who preaches that the natural number of personal electronics devices is two? What about that? Huh?

Just let it go, OK? Nobody likes a smart aleck.

In all seriousness, holding yourself to two portable systems remains a valid use-case model. A tweener phone like Samsung's Galaxy Note series paired with a chameleon-like Ultrabook such as Lenovo's Thinkpad Twist or Asus' Taichi can make the two-device model compelling for consumers who want to travel light.

[ There are plenty of tablets and smartphones on the market. Does Microsoft Really Need To Make Its Own Hardware? ]

If you buy into the logic behind limiting yourself to two devices -- I still do -- then doubling the number may seem counterintuitive. But it's not. You can still lighten the load with four devices in your quiver. Remember, tablets are siphoning activities from a swath that's far wider than just our smartphones and laptops. They're pulling in books and magazines. Portable gaming systems and DVD players. Camcorders and point-and-shoot cameras. Oh yes, and pads of paper.

Think of it this way: what would you say to someone who asked which you prefer: smaller 5 x 8 pads of paper or 8.5 x 11 pads? You'd likely say, prefer for what? Right?

We can do the same tasks on either size. But most of us would rather jot notes -- shopping lists, agenda items and other reminders -- on smaller pads, and turn to the letterhead-sized pads for more comprehensive tasks, such as meeting notes and design ideas.

It's the same for our battery-powered devices. We can browse the Web on our smartphones -- and we'll turn to them for quick fact-finding searches around town. But we're far more likely to hop onto the laptop if we're doing more complex, detail-oriented activities.

Google released a study in August which found exactly that: When consumers have a choice, they're more likely to turn to the display that most closely meets their needs. I know I'd rather read a book on a 7-inch tablet than a 10-inch device. Interestingly, the study's publication was sandwiched between the release of Google's two branded tablets: the Nexus 7 and Nexus 10.

I invariably hear a couple of objections to the four-device model. The first comes from people who suffer from device overload. They just refuse to lug all those devices around with them. That's fine, don't. The 7-inch tablet, loaded with a book or two, would tide you over for a weekend escape.

The second issue has to do with cost. It's a valid point. I could argue that, from a TCO point of view, a 7-inch tablet is a whole lot more cost-effective than a wall full of books. But if money's tight, that argument falls on deaf ears. Buying a tablet demands too much of an upfront investment, like paying for the paper today for all the books you'll read in the future.

Here's an argument that plays better: buy used devices. The market for second-hand tablets is vibrant. If you can force yourself to get by with a last-generation tablet, you can pick a pair up for less than you'd spend on one new device. Consumers who have to have the latest thing are replacing their tablets more quickly than even their smartphones, so there's plenty of supply. And a bonus: Consumers typically treat their tablets with more care than their smartphones, so they're usually in pretty good shape.

However you decide to overcome these issues, it's important that you do, even if you don't intend to buy two tablets today. Because you will want two. Soon. You'll see.