Rainbow tables

Other commentators have observed that Rainbow tables have been "eclipsed" by the rapid progress of GPU-based parallel brute force attacks for most scenarios where they would once have been viable.

Rainbow tables are a time-space tradeoff, RAM and disk space continues to be only marginally cheaper than it was a few years ago, while GPU based "brute force" attacks can be thousands of times faster than CPU based attacks from a few years ago. So rather than buy 50TB of storage for rainbow tables targeting a single algorithm, it is more tempting for attackers to buy, rent, or indeed steal (through a botnet) a dozen mid-range gaming GPUs and have the flexibility to attack dozens of different algorithms for which GPU-based brute forcers are available.

Salt does still mean the attackers have to attack every individual password, rather than gathering a list of the most common hashes and attacking those for disproportionate gain, and it also means if some day rainbow tables or a newer time-space tradeoff would otherwise have been viable that's sidestepped. But it's far less important today than it was a few years ago.