You don't need to 'set up' anything for your LiveJournal account to function using OpenID. It's exactly like if you want to buy something from a site that you know you don't intend to return to so you don't want to create an account for that site and just log in using your Google account instead! When we import comments (and community entries) from LiveJournal, we show them as having come from LiveJournal, so people can still manage them with their LiveJournal accounts (by logging into DW with their LJ account).
The reason to claim your LJ OpenID with your DW account would be to make sure you can still manage those imported comments and community entries without having to log into LJ. For example, if you made a post to a community on LJ in 2005, and that community imports all its entries to DW, the DW copy of the post would look like it was made by katuah.livejournal.com (or whatever your LJ username was). If you wanted to edit or delete it, you'd have to log into LJ, and then use your LJ to log into DW. If you claim your LJ OpenID with your DW account, that entry would look like it was posted by katuah, and you'd be able to edit or delete it with your DW account.
I know this is a bit confusing, so an example: here is the post I made to my LJ account first announcing Dreamwidth. That post was made on LJ, and I imported it to DW. The first comment thread on the post shows as having been made by zorkian and synecodochic: both Mark and I made those comments on LJ, using our LJ accounts. We both claimed our LJ OpenID accounts with our DW accounts, so now we can manage those comments while logged into our DW accounts if we want to. The second comment thread was made by svmadelyn. They haven't claimed their LJ OpenID with their DW account, so it shows as having been made by svmadelyn.livejournal.com. If they wanted to manage that comment, they'd have to first log into their LJ account and then use their LJ account to log into DW in order to manage it.
If you don't intend to delete your LJ, the only reason to do it would be convenience! But it's a good idea anyway, especially if you participated heavily in communities on LJ, just to make sure you'll always be able to control the imported DW copy of comments/community entries that you left ages ago.
no subject
The reason to claim your LJ OpenID with your DW account would be to make sure you can still manage those imported comments and community entries without having to log into LJ. For example, if you made a post to a community on LJ in 2005, and that community imports all its entries to DW, the DW copy of the post would look like it was made by katuah.livejournal.com (or whatever your LJ username was). If you wanted to edit or delete it, you'd have to log into LJ, and then use your LJ to log into DW. If you claim your LJ OpenID with your DW account, that entry would look like it was posted by
I know this is a bit confusing, so an example: here is the post I made to my LJ account first announcing Dreamwidth. That post was made on LJ, and I imported it to DW. The first comment thread on the post shows as having been made by
If you don't intend to delete your LJ, the only reason to do it would be convenience! But it's a good idea anyway, especially if you participated heavily in communities on LJ, just to make sure you'll always be able to control the imported DW copy of comments/community entries that you left ages ago.