Does the long sales copy letter (on the web) have a place when you're selling a SASS app? Say an appointment reminder system? I've been convinced it works for

Does the long sales copy letter (on the web) have a place when you're selling a SASS app? Say an appointment reminder system? I've been convinced it works for info products -- but haven't seen good examples for applications. (or is the long for content done only in emails and not on the site itself)