Earlier quoted context omitted.
I have an ASUS PN40 that idles at 1.8W (_at the wall_). This is with one SSD and LPDDR3 memory.
Yeah the small systems are often very efficient, but you can't really use them for file serving from HDDs. USB (and even eSATA) enclosures are not a suitable replacement for internal SATA ports. Their controllers & firmware are not stable enough for heavy concurrent use and often crash or become non-responsive. Idling an old Xeon or a 12100 at 4W is also a bit more challenging than a 1.1Ghz Celeron.
If you want a shitton of SATA ports you obviously have to go somewhere else but then even the power consumption of the idling SSDs will start to be troublesome (and when in use will likely necessitate another PSU).
I also disagree with the assessment of eSATA (since it is basically external SATA), and think the claims of problems with USB to SATA controllers are very exaggerated. They all have terrible power management though; if I had to use one, I'd just cut USB power entirely on idle, which would btw also help with any controller issues.