No, I didn't say or mean to imply it's a way to reduce RAM usage, though for AVR users that's an ever-present challenge.You are describing the corner-case where you don't have enough memory to keep a SPI class instantiated
I was addressing the problem of "time sharing" a resource. An SPI port can be time shared as each use has a different SS pin. But as has been debated here, there's no means for mutual exclusion for this among multi-authored code/libraries.