having ability to edit doesn't mean you HAVE to do "substantial writing".
how often do you fire up MS Word to start writing a whole document vs making minor changes on existing ones?
have you ever received calls and emails from other people telling you that the document you just sent needed some minor corrections? do you want to be able to correct them on the fly on the iphone or go back to office, boot your PC and make the changes?
also, excel is very usable on a mobile platform. you can create a lot of mini spreadsheets to help you perform a lot of tasks. you can use it to keep track and plan your expenses, calculate your investment portfolio, or simply do calculations that are not easy to be done using the simple calculator app. when i went shopping for a house i created a little spreadsheet to calculate the monthly payment I would have to pay in different scenarios (price of the house, amount of down payment, interest rate, etc) i plugged in the numbers and i got the answers right away.
i don't consider any of these "substantial writing".