Because (going by scheme271's numbers) I might not want to waste lots of expensive RAM on something or mess around with offline deduplication, when a less resource-intensive alternative is entirely sufficient for whatever my use case is.

Maybe it's because I've been recently playing around with deduplicating subtrees in a different domain, but I don't see what makes this feature too complex to be worth the cost. Sure, symlinks and hardlinks require some additional care compared to plain files, but it's easy enough to add tests for those cases.