Is there any point to not using 6 champions?

I was looking for a new deck to build and I came across a spider aggro deck (on Mobalytics) which used 3 copies of Elise, and no other champions, why wouldn't you use another champion just for damage?

This especially comes as a shock to me, who has been using an Mf/Quinn deck, which, although it has helped me climb quite a bit, is completely useless if you dont draw any heros.