It may be beneficial to keep the e-mail list as well. We did lose some participation from a few members who can't or don't want to use forums. There are also a few folks who can send e-mail at work, but can't view web sites. And there are a few RS members who just seem to have problems every time they try to log in to the forum -- so they can read stuff, but never post. Plus they can't see the "Members Only" section, since they can't login.
We ran the two simultaneously for about a year. The e-mail list was my baby -- I started it in Jan. 2001. We killed it about a year ago.
Personally, I love the forum!
If it's not costing you anything, then you might want to keep it around for your forum-impaired members.