I think most people have the potential to be a brand evangelist if it's a product they really like. Some may be less enthusiastic than I am in this particular example. I don't know if it would be ideal for a company to consciously decide to focus on a set of customers that have a propesity to be a brand evangelist. Think of the friend that's always gushing -- it's almost a case of too much endorsing. After a while their opinion means less.
The best brand evangelist is one who, for some reason, is completely taken with a few products and can't stop talking about them. I can't think of a reason why a company should decide on a limit for these kinds of customers -- nothing comes to mind as to why it would be a bad idea to have as many enthusiastic customers as possible. I'll have to think about that.