well, as a user, there's no real way for you to know that aptitude purge completely removes everything, you have to trust that it works. Sure, as a power-user or developer, you can go look around /etc or /usr/share or wherever and remove all those configuration and documentation files, but will you?

I used debian for about a year, I trusted that aptitude purge removed as much as it could. I knew that trust was misplaced, but for my purposes as a user, it was good enough.

Similarly, for iOS devices, I don't know how well it removes config files and whatnot. I simply trust that it removes everything. I have slightly more trust in iApp config files being removed because these apps are sandboxed, and it's a relatively simple matter of removing the directory the app fills.