Acorn didn't do the PC Cards to impress outsiders so much as to impress existing Acorn users and coz it was cool and coz they could. Worked for me. The 'little effect' statement kind of negates the fact that hundreds of these cards were sold to people. I think you're more upset because you were more interested in multiple ARMs than PC cards and Acorn didn't deliver this.
Someone can correct me if I'm wrong but the 2nd processor slot on an A540 would have been for a FPU.