While you are correct in stating that 2.5" laptop hard drives (spinning platter type) can be used, this thread is focused on SSDs. Moreover, 2.5" drives of the spinning platter type are hardly more inspiring than the 3.5" variety. They can be easy found anywhere for cheap prices, but so what? SSDs are what the doctor ordered for lowering power consumption and heat and noise.
And as to "adapters" I covered that my previous posts here. Why get an adapter if you can get a 3.5" form factor SSD drive that is already SCSI-1, SCSI-2 or SCSI-3 (narrow) and is drop-in compatible with the stock hard drive? The price would be the only real consideration here. But SSD prices today are the lowest they have ever been. It just depends on how stingy these companies are in clearly out their "dead stock" of older model SSDs (older models, yet perfect for our vintage Macs, with speeds that more than saturate the original SCSI busses).
And as to "adapters" I covered that my previous posts here. Why get an adapter if you can get a 3.5" form factor SSD drive that is already SCSI-1, SCSI-2 or SCSI-3 (narrow) and is drop-in compatible with the stock hard drive? The price would be the only real consideration here. But SSD prices today are the lowest they have ever been. It just depends on how stingy these companies are in clearly out their "dead stock" of older model SSDs (older models, yet perfect for our vintage Macs, with speeds that more than saturate the original SCSI busses).