Forums have the advantage of being easier to search for those just observing the conversation.
How does that differ from using NNTP to access mailing list archives through a service like gmane? My local email and news client has no problems searching through past messages and is faster than an online forum is.
Having a known viewing platform also has it's advantages - when using email one has to assume text only.
Usually forums have a subset of markup that's available and it seems to differ from platform to platform (i.e., how do I use bold, italic, or underlined text). For a text based mailing list or newsgroup, the standard was *bold*, /italic/, and _underline_. Some clients would even support that type of informal markup when rendering plain text.
The switch to a forum is a good idea so long as email alerts / updates to new content are provided.
It would be nice if they could set up a NNTP server with an email to NNTP gateway for those who want to continue using email and allow for participants to register for accounts to post to the group. It could peer with gmane for archival purposes.