It's perfect because you can lower E to D when you need to. Don't get me wrong. My comment was made tongue-in-cheek. I play both 10 and 11 string guitar. After doing so I am left with the impression that there's a reason that the 6-string is the predominant instrument. It's really the perfect number of strings. I look at adding more strings like adding more salt to a recipe. A little salt making a dish taste good does not mean more will make it even better. When you add more strings, at the same time you give up something. So I like mulit-string guitars, but I really appreciate how perfect the 6-string is now.
Yes, 6 is perfect, because it is the minimal number with which you can play a huge repertoire. Actually, to me it's amazing what can be done with just 6 strings. Still, multistring instruments have an appeal, don't they?