It doesn't need to be a kit for you to know everythng about it. I could take my rtr rustler apart with my eyes closed easily, and yet have a harder time taking my rc18t FT apart(also with eyes closed) even though the 18t is soo much simpler and I built it. This is just because I have had so much more experience with my rustler than my 18t.
I believe you get more experience from buying an RTR and working on it than you do from building a kit once. The kit also comes with instructions on how to build it while with the RTR you have to KNOW where everything goes and how it works.
This is a very opinionative subject and it's not just black and white... These are just my thoughts on the subject.