There's lots of them around, and most times I comment asking the OP to edit the answer and add more detail. However there are some cases where the OP isn't around, but the answer is still valid (after reading the link) or they're new and could use a little guidance of what a good answer should look like. In these cases, I typically add some quotations from the linked site, and any pictures that may be relevant.
Is this the correct practice of editing link only answers?
To make this as precise as possible: YES, please.
If you have the time to check the linked site and distill the most essential parts, then post them here, you've done us a big favor. First of all, you're making the answer more useful, and you can show others how it's done – leading by example. In the long run, people will see these edits and know how to post better answers.
I don't think that downvoting and/or deleting those answers always makes sense. If it's easy enough for you to find the most important passages and post them here, then just do it instead of going to the trouble of having a possibly helpful post removed.
If the post is not salvageable though, the link is dead, or it'd take you hours to figure out how it solves the problem, flag it for deletion as not an answer.
