there’s always a trade off between speed/convenience (ie more speed, less convenient) and simplicity/power (ie more simple, less power).
hard coding changes into core files will make for leaner code, but as mentioned earlier, you will have to keep tab of all mods for all the core files you changed. a pain when upgrading if you’ve made 10 changes to 10 different core files.
remember also that core file logic and structure tend to change during upgrades. (the main reasoning behind plugins is so you don’t have to touch core files)
this is where kca mods make life simpler, because kca deals with the finished product (ie. the page content), which rarely changes in between wp upgrades. so you only need to create a little snippet of client side code for the desired effect that should work through upgrades.
kca is not only for quicktags as you can see. it is quite a powerful tool. however, to make the most of it you would probably need to know a little about client side scripting (javascript/vbscript/dhtml/dom/etc), or you can just cut & paste code you found into kca.