Although they are independent applications they are needed to keep the minimal desktop functional. As far as I know this is why they are needed (or at least why I undestand their are need).
KWrite and Konqueor textfile viewing use Kate embedable KPart. Kate KPart is part of the Kate library that is probably 95% of Kate so it's there to complete the 5%.
The control center at least needs to run things in superuser mode. I suppose that kdesu is there to do the job.
Well, if you see any "Print" menu entry somewhere in the desktop then you probably need kdeprint to do the job. It's not like "if you don't have a printer then no Print option is shown" (It would be nice though).
konsole, hmm, I think this is what puts "base" in "kdebase" ;-). Really I suppose that options like "Run in terminal window" have dependencies in konsole.
kpersonalizer is run once in the first session but its needed to have the KDE Desktop, as presented to the work, functioning and is a good thing for new users.
kscreensaver, well it probably could be left out but I think this is more or less like kdeprint. Control center allows to configure the screensaver, the desktop menu allows to configure the screen saver. I guess that its a given component of the system and not a plugin.
ksplash, how could you wait the kde startup without this little wast of memory and diskspace ;-). Is like kpersonalizer.
Well I din't mention kaddressbook because I really don't see why it's needed at kdebase but suppose that is something like kate. The library is needed somewhere and the whole app is cheap compared to the library only.
Well, I din't say anything usefull here, just my opinion so I would like to add one more thing.
I don't thing that this division of kdebase, pim, edu, etc is final. Things evolve do do dependencies. When everything works well, dependencies are perfected, meaning that commom parts a concentrated or in other way reused and all the components gain a more solid structure. Even the Qt framework is trying to work on their dependencies and separate UI from Qt core. So expect KDE to suffer similar changes.