I believe there is quite a benefit. You should put a robot file in there too so that they do not index all those PHP files unnecessarily. The main thing I can think of for you to put in your forum site map is
forum
topic
images (maybe)
index
maybe something like this
<?xml version="1.0" encoding="utf-8"?>
<urlset xmlns="http://www.google.com/schemas/sitemap/0.84"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.google.com/schemas/sitemap/0.84
http://www.google.com/schemas/sitemap/0.84/sitemap.xsd">
<url>
<loc>http://www.YOUR-FORUM.com/index.php</loc>
</url>
<url>
<loc>http://www.YOUR-FORUM.com/viewforum.php</loc>
</url>
<url>
<loc>http://www.YOUR-FORUM.com/viewtopic.php</loc>
</url>
<url>
<loc>http://www.YOUR-FORUM.com/images/</loc>
</url>
</urlset>
I would block all that other stuff (of course your SM code will have different files, I'd imagine)
User-agent: *
Disallow: /admin/
Disallow: /db/
Disallow: /includes/
Disallow: /language/
Disallow: /templates/
Disallow: /common.php
Disallow: /config.php
Disallow: /faq.php
Disallow: /groupcp.php
Disallow: /login.php
Disallow: /memberlist.php
Disallow: /modcp.php
Disallow: /posting.php
Disallow: /privmsg.php
Disallow: /profile.php
Disallow: /search.php
Disallow: /viewonline.php