My DD (6), has her own netbook (which she paid for herself) and which she uses mostly for school work, but also for going on Cbeebies.
DD is quite academic (she is G&T at school, so bright, but not amazingly so) and she often likes to find out more about the projects she studies in school. We struggle to find age appropriate books for her. I do have a collection of non-fiction books at home which she uses, but our local library is a mobile van which comes once a week, and actually is quite dire. So save buying tons of books, the internet serves quite well. For example, recently, she was doing a project on India. We had one book on this at home, and no library visit for 5 days. So we found a school website on India, and DD spend a good half hour reading through it and learning new things about India and Indian people.
Furthermore, she most frequently uses her laptop whilst sat next to me and I'm working on my work laptop. Our Desktop is in the study, away from the rest of the house and I didn't want her to use DHs very expensive Apple Mac where she couldn't be supervised, so that is why we got her her own laptop.
Incidentally, she is also a talented Gymnast (trains 4 1/2 hours a week), so I have no concerns over her posture, and as she is G&T, I also have no concerns that it will make her unable to read books.
I think every child and every situation is different. Whilst a laptop might not be appropriate for your child, in your situation that does not mean that it is not appropriate for another child. Every situation is different.