~ do you guys think that only those who write (and therefore understand all the problems and issues that writers face) should beta? ~

I actually think that a more objective approach can be useful. And just because someone is not as imaginative or articulate as someone who writes, that doesn't mean they don't have a good grasp of story structure, and for a beta that I think that, it's that which is most essential (along with knowing your grammar and syntax). So no I don't think only writers should beta.