Yeah this is true, but it works for enough websites to be a meaningful option, and it is almost always going to work better than something you have home-rolled (unless your core product is a direct competitor or something, in which case all bets are off ;))

A good is example of this is Pinboard which claims to offer website archiving. A friend has over 100,000 links saved (with an archival account) there. When we spent a few minutes looking at the archive links for those items a few weeks ago, we couldn't find a single correct, working, accessible archive from the most recently archived links (listed as archived 5 weeks ago, so also not up to date).