Seth Godin has an insightful blog post up where he very succinctly describes the fundamental difference between spam and permission-based email campaigns. 

A spam campaign feels like a smart idea, but over time, the more you use it, the less your brand is worth. A permission campaign, on the other hand, only grows in value, until it gets big enough that you can build an entire business around it.

Earning permission is a long-term, profitable, scalable strategy that pays for itself. Think about how much better off a brand would be if it took the time to make promises, keep them and be transparent about its communications.

Ultimately your list is only valuable if your subscribers are engaged.  An email list of 10,000 passionate, engaged people who have truly opted in is more valuable than a list of 100,000 addresses acquired through nefarious means.  Those 10,000 folks will buy your product, write letters to Congress and/or donate money.  With spam lists, you’ll be lucky if they even open your email.

Godin’s full post is worth reading.