It's really not complicated to comprehend...

John,
It's very simple. It's part fad, part trend, part hype, but there is method to the madness. First off, just because there's an iPad for $1000 doesn't mean you HAVE to buy it. You can buy perfectly acceptable tablets running Android that will do just about anything you can do on an iPad for $70 USD.
As far as the mobility factor goes, a 7" tablet will fit into a larger pocket. I have a vest that has an internal pocket and my tablet fits it like a glove. It'll fit into my pants as well.
What's it good for? Plenty of things. I can take a quick note using a freebie notepad app. Works great when I have to go to work at a clients and need to monitor my hours. A few keystrokes won't kill you nor drive you up the wall. Writing a book like "War and Peace" - yeah... That will get on your nerves.
I also use it for surfing the web. Specifically, my local supermarket has online only deals and coupons that I use the tablet to access the site with. I can pick and choose the deals I want - and it also functions as a digital shopping list. I log into the store's free wifi and look up whatever I need.
It's also saved me at least $10. I buy stuff and on occasion, the digital coupons and deals don't always register or expire before their time at the store level. But I just whipped out the tablet, showed them the deal on their own website and I got the discounted price.
Keep in mind, this is a wifi only tablet that has no built in 3G or 4G which is fine and dandy. I don't have to pay an additional bill for it. It's mine, I own it free and clear.
Additionally, you can also play games - like the viral hit Angry Birds - and the various incarnations of that game.
So, yes, for light duty things, a tablet can do plenty for you. It's not that laptops aren't portable, it's just that tablets (especially the smaller 7" ones) are MORE portable. Is it a replacement for a desktop or laptop? Of course not! But it IS a handy tool for when you don't want to lug that 10 lb laptop somewhere - like grocery shopping. I still do 99.9999% of what I do with computers on my desktop, and that isn't likely to change anytime soon.