I've been deleting my content on Reddit, and while I'm sad to see some of it go, I would not be happy to find it's been copied over to Lemmy without my permission. Also, a lot of what I post on any forum is meant to be part of the conversation happening when I post, and having it sit around getting stale for 10+ years isn't necessarily a good thing. If people liked what I wrote and wanted to hold onto it in their own records, that's fine, but that's not the same as migrating my content to a different platform, context, and audience. Reposting takes control of the content out of my hands, and puts it in theirs, and possibly makes my content benefit people I don't like. (Fortunately, it is extremely unlikely anyone does want to repost my content, lol).

As far as whether it's good for Lemmy... I think it depends on whether or not we want to recreate a subreddit. Putting up the same content as a particular sub is going to tend to recreate the vibe of that sub. IMO that could be either very good or very bad, depending on the sub.