There’s a variety of good reasons to let your readers email you, and I think it is something to encourage. There are times when open comments might not be suitable such as:

1. There is an issue with what you have written. They may think you have stolen something, they may think the information you have given is misleading, they may simply think you are an idiot - they are hopefully wrong on all counts, but far better a private email to discuss the matter, than a public comment!
2. Your content, or you yourself may be being abused elsewhere on the web, and they wish to inform you without drawing others attention to it.
3. Networking - I’ve had several interesting offers and opportunities in the past, as well as simply the mechanism to chat with my readers in a private situation - it’s very useful.

I’ve had reason to email other bloggers, but have given up when there was no means of private contact, so I think it is essential to provide an email address. For spam reasons it’s a good idea to create one specifically for each blog you have, and use it only for matters relating to it. I use Gmail, mainly for it’s anti-spam & large storage capacity, and have found it to be excellent. I believe it’s now moving to completely free access, but if you still need an invite, let me know as I have plenty.

Finally, you should publicise the address: Some people prefer using a contact form as that helps cut down on spam a little more, but I prefer to just have it on the front page & let people see it and use it as they wish. If you like the little Gmail icon on my left sidebar, you can get one generated ( as well as for Hotmail, MSN, Yahoo! and loads of others) at the Email Icon Generator.