I do keep out of stock product pages active for the very reason you mentioned: it's more unique content for the search engines to read, and also, if someone is searching for my out of stock item, I would still want them to find my site because it is very likely we would have a similar alternative option of which they may purchase instead.
If anything, our product pages always include a ton of information, including PDFs and pictures, that just seem helpful from a consumer's POV. Even if they do not end up purchasing said product from us, they can still research it with us!
It really thus depends on the nature of your website and your products, but there is great value in retaining unique content, so if your product page is filled with useful product information, I'd say definitely keep it available for the search engines and consider linking to alternative in-stock options for your visitors to pursue as well!
