There is a middle ground - either an Intel Atom or Intel Celeron based server - typically use around 10w and you still get niceties like SATA and even PCIe
A long time ago, I remember there being desktop MBs for the Pentium M.
Most desktop motherboards won't allow the PCI express bus to sleep for example, using an extra ~4 watts, $8 per year in eco-friendly countries. Across the ~10 years your NAS will probably sit in a cupboard, that's $80, which isn't an awful lot, but probably would have let you buy a bigger SSD or hard drive which would have had more utility.
However, for running random VMs, etc, I think a laptop is quite a good proposition.