Alex Jurgensen at Apple World Today makes a great argument for keeping the Mac's startup chime:
A use of the startup chime that may not be widely known is that it is used by blind and partially sighted users to determine if the computer is booting up. As a legally blind Mac user, I have found this increasingly important since Macs have become quieter due to improvements in hardware such as solid state drives and the removal of optical drives.
Although Apple's reasoning makes sense on the surface, it leaves blind and partially sighted users in the dark as to whether or not the computer is booting. A better approach would be to restore the startup chime but provide a easy to find setting to disable it in System Preferences. Perhaps Apple could rename the "Startup Disk" preference pane to simply "Startup" to accommodate this new setting. While it is possible to reenable the startup sound, the fact that it is disabled by default on the MacBook Pro makes the job of supporting the Macs of others more difficult for blind and partially sighted users. We therefore ask Apple to bring this iconic and highly useful sound back.
I've wrote many times about the value of multiple sensory cues for people with disabilities—Jurgensen's request is along those lines. For blind and low vision users, myself included, the startup chime is a cue that my Mac is powering on. It's an edge case scenario, sure, but as is true with everything accessibility-related, the tiniest of details add up to make the biggest difference in terms of a positive experience for people with disabilities.