There may be some members whose bookmark takes them to the old site every time, whereby if they even touch the site they are shown as logged in for 15 minutes or so. Maybe they are too lazy to update their bookmarks.

We have a lot of valuable links in Google which currently point to the old site. One of our aims is to provide free information for Omega Owners so when they search in Google it's probably preferable that they don't get a load of dead links, or nothing, as Google start to realise most of the links to omegaowners.com are dead. In the fullness of time Google will have indexed the new site and this will be less of an issue than it is today.
We also have links within the new forum to the old forum - where someone posts a link to a maintenance guide when answering a general help query, for example. Do we want these all to go dead? I don't think so. Not yet, at any rate. Once they are a couple of dozen pages down in the history that might be less of an issue... Of course, in the meanwhile, as soon as a previous member of the Yabb forum clicks on one of these links, they are shown as logged into the Yabb site.

I can see the argument for keeping a read-only yabb site for the moment.