There are two problems in restricting access from Yahoo! and Hotmail email addresses:
The first is that it would limit a lot of honest users. If we allow anonymous contributions, marked only by IP address, we can surely allow people with any email address to contribute.
Secondly, doing this would not hinder some people. For example, I can create any number of email forwards in my domains in a few seconds. True, you can block the entire domain, but there are some domains with many legitimate users.
Besides, this isn't what Wikipedia is meant to be. Identity verification is all well and good, but not when there are so many innocents involved.