Oh whoops, I guess my VLSI professor lied to me. Maybe it only happens with certain multiprocessors. But yeah, there are definitely disabled cores in a lot of processors for the reasons you mentioned.
https://en.wikipedia.org/wiki/PlayStation_3#Technical_specif...
The cell processor had a main PowerPC core and eight floating-point SIMD co-processors called synergistic processing elements (SPE). For the PS3, one of the eight SPEs was disabled and another was reserved for the operating system, leaving the other six for developers.