I think it all depends on your site. i'm using my forums as simply an "engine" for the main website, so a "portal" or site is very important to me. Another example would be
TS3 where more than just a forum is offered, there's extras for the site.
However, if you just have a forum and that's all you intend to have, go portal-less - such as
OTC. A portal/site would be silly for this forum because it is just that ... a forum.