Read only docks are a specific tool for specific uses. Use scenario 1) information is stored on the disk from a known trusted source. This information is desired by someone with a computer you don't trust and can't confirm it's security. Read only dock allows local data transfer in a single direction without damaging infections making it to the hard drive in the dock because it lacks the physical pathways allowing write access.
Use scenario 2) forensics. Forensics is more than a legal examiner searching for kiddie porn on a perp's hard drive. It also takes the form of known clean software being loaded from a drive with hardware write paths severed. This can be used with debuggers, memory dump software, file scanners of various sorts, and, yes, the more common malware scanners. These uses are all valid and since time is money, the faster this can be done, the less time is spent on each case.
In short, this sort of device isn't for the average kid writing backups at home. It's a device for professionals needing an inexpensive alternative to read-only imaging hardware. The last time I checked the least expensive of such was $250, and it was extremely limited.