The alternative to the register banking would be the possibility to address more than 8 registers with the same opcode. But this isn't possible when only using 8bit instructions.

Look at the opcode of MOV Rn,#data instruction, for instance, which is:

0111 1rrr

the first 5 bits define the instruction and the last three bits "rrr" define the register. Addressing more than 8 registers would mean, that only 4 bits could define the instruction. As consequence only 16 different instructions could be implemented.

Register banking allows you to address more than 8 registers without the need of using two byte instructions. Only one register select at the beginning and the end of a series of instructions is needed. Then, fast one byte instructions can be used.

Kai