There are that many reasons for someone to buy a mini netbook, the most popular one though is getting some content creation done. Since the processor used in mini notebooks is not the fastest nor it comes with the most cores, said content usually needs to stay between boundaries of the office suite.
If you were thinking about getting a netbook for word processing, good news is that you'll have no problem running a word processor. Microsoft Word however costs money and I've yet to see one computer that was sold with a full license.
Some people say that netbooks are just mock-ups of a normal notebook, and are not good for anything else than browsing online content, writing and reading e-mails. Fortunately this statement is as far from truth as it can be and I'm the living testament for that. I've been using my netbook for more than six months as a primary computer and let me tell you that it's been a great help to me.
It has a word processor, comes with Word 60 days license pre-installed (along with other parts of the Office suite), but you can use anything else you like. I used OpenOffice Writer for some time, now I'm typing on Q10 full screen text editor.
Other than the software pack netbooks need to have another part to compete with full size notebooks. The keyboard is rather stuffed because of the small external casings. If you have extraordinarily large hands, you may run into problems with one, but a quarter inch over 6 feet and I have no problem at all. In fact I can type somewhere around 70 wpm on the keyboard of my Asus seashell design netbook. 1005HA, 1005PE, 1001HA and a few others of the 1xxx series come with this design, if you want a good set of keys, I suggest you try these for yourself.
Another one I liked was the one used in HP Mini 210. On Acer Aspire one I found the effective surface of the keys rather limited due to the design being similar to that of a normal size notebook, only smaller. If you take only one suggestion I make, go and try a few netbooks for yourself before spending any money, the safety of your joints and fingers really deserve that much time.
For word processing I could not imagine a better solution than a mini netbook, because they run for a long time on a single charge, and with the a weight of 2.5lbs they're hardly a challenge to carry around for a whole day.