Being able to hotswap more components in the data center without power cycling the server is becoming more useful again (I say again because this was a long standing benefit of mainframes). More than just hard disks, it's useful for SSDs, and now GPUs and other types of accelerators. PCIe supports hot plug, but it's rarely actually used.
Not just mainframes, there was and is even RAM and CPU hotplug x86 hardware (but rare and expensive). Sun hardware (enterpricey, but not mainframe per se) had CPU boards you could pull and replace while running. Network cards and HBAs are other more typical examples of things you could hotswap. But the trend has favored software based solutions providing redundancy on multiple cheaper servers. Imho rightly so.
