Yup kit lens is enough for most of us. But compared to a pro F/2.8 zoom, you are missing
1)F/2.8 = Better bokeh
2)F/2.8 = More light enters the camera, thus a brighter view finder
3)F/2.8 = Allows the camera to auto focus in even lower light conditions.
4)Faster autofocusing (in most cases)
5)Much more weather resistant(in weather sealed pro zooms)
6)Better build quality=higher chance of working after drops. (something a kit lens cannot take much)
7)Nikon's gold band or Canon's red band
8)Assurance it would work for at least 5 years.(most non-lemoned good quality lens)
For a small handful of us who loves ultra wide angle to play with perspective or certain genre of photography like birding, you cant use a kit lens for that, even a 1000mm mirror lens isnt enough or so my friend say.
There are many genre of photography that requires special lens like macro,birding,sports. Its important to know what you want so that you dont spend unnecessary money. Personally I like ultra wide and ultra tele, but I do events too, so currently i have a f/2.8 zoom(tamron sadly)and a lens that give me 450mm equivilent on 35mm film. But saving up for an ultra wide. In my case i wouldnt go near a macro lens, but thats just me. Hope this helps you.