Why? Concerned about spam?
That is the primary reason. I have no problem with SPAM because of my diligence in this area. In addition, I do have secondary email accounts when an email address is needed.
The other concern is privacy. What I get in return has to be worth what I pay in loss of privacy. For example, I did trade personal information for a free EFax account for occasional incoming faxes at home. There are enough free newspapers on the web that I don't have to pay with information for the few that require it.
I realize that I could easily lie on the registration forms; I was just raised to be too honest, I suppose.