Well even for the simple uses you describe the iPad is pretty poor beyond very basic or straighforward document (or ppt) writing.I don't doubt that an iPad can replace a laptop for many people. A lot of people just need to be able to type up documents and occasionally make power points. Which an iPad meets.
However, an iPad is still hindered by iOS. The file system still needs a lot of work. Also not being able to download and instal 3rd party software is a huge issue. You can only use the app store to download applications.
And for me as a teacher there is software ( software such as active inspire) I need for my lessons and plans that aren't available on iOS or android platform. Being the ability to have multiple windows at one time definitely makes my work flow faster. I can't do that on an iPad.
That is assuming people don't write documents or do presentations out of thin air and need to prepare data, graphics, tables, photos or pictures.
Some times you can do that stuff on iPad but it requires more work that it is worth. Better to just take out a notebook.