I'd like to suggest 10 reasons why maybe you should:
1. You took time to write to me. (if it's important to you it then might also be important to me)
2. It would be rude if I didn't (especially if you are a potential/existing customer or Business Partner)
3. Email is our primary business tool for communications
4. You might have a good suggestion
5. You might be a customer who needs help (and if you are, "my rules" about why I won't read/reply, may not mean Jack %&$# to you)
6. It only takes seconds to say 'sorry I can't help' or 'no I'm not interested' (Try it - It actually works!)
7. If you spam me, I pass that off to our spam filter - it won't happen again
8. I tend to turn off twitter, facebook and other interrupters for extended periods of time during the day
9. I only check my inbox a couple of times a day, so it tends not to interrupt my workflow
10. Its OK to collaborate.
Anyway just food for thought.