Well if you want to invest in an ico or an existing coin it would be a good idea to read it and i always do. I know people can just buy a whitepaper or take i from another project but it is a pretty good guideline. But i think most people who are investing wouldn't understand half that is in it.
Do you ever need to read whitepaper for existing project? Example etherium? I dont think so.
As i know, read whitepaper is important but only for new ico project but not for existing project.