You believed in a coin because it was always on top 10 coins? Well that's a great way to value a cryptocurrency..
Yes it is. I would be much less inclined to trust something that had arrived in the top 20 or 50 one week ago. The chances are it might be gone one week later.
True, but NEM has been in that range for several years - it isn't a new project after all. Also, top X gives more visibility, which means people can do their homework and learn about the project. For example, there may be a great project today between top 300-400 but it would be very hard to find it if you're just looking at CMC.