Sometimes with factory coils, if you flood them or accidentally burn them (easy with these new cotton ones) then you have to toss it. You could mess around with rebuilding it, but its harder than a real RTA. Dry burning and replacing the cotton on my kayfun takes a little over 5 minutes and I do it once a day. However, if something goes wrong with the build, like if I just don't like it or want to change it for some reason, it only costs me another 5 minutes. Each time you do that with a pre built, its a $2-$5 fee. Factory tanks are like cells phones and the coil heads are the subscription.
Also, there are new RTA's out now that are cheap and amazing performers, like the one here:
https://www.youtube.com/watch?v=xGbmwv1coYY
Eleaf Lemo. Its been all the craze lately because it performs like a damn dripper. I don't have one, but you might check it out. I'm never going back to factory clearomizers. The RTA world is just too good.
I forgot to mention, the benefit of an RTA, for me specifically, is being able to change the wick daily and dry burn the coil for no cost. I use the same coil for weeks sometimes. This keeps flavor perfect all the time. No cross flavor contamination and the coil doesn't have too long to get gunked up.