Argh. More cookie issues.
The Z80 in the MD/Genesis can be used for audio or as a general-purpose processor. The YM2612/PSG are both accessible from the Z80's address space, so the Z80 can control the audio subsystem. But all the 68K is doing to instruct it is writing it standard operations to perform, so you could just as easily run game logic code, as was suggested, on the Z80 and pass the results to the 68000. However, this means the 68000 must personally send the relevant instructions to the YM2612 synth and PSG for audio to be generated. Or the GLU chip can be flipped into 'SMS compatibility mode' by changing the voltage state of a cartridge slot pin to use the Z80 as the master processor (the VDP and PSG in the MD/Genesis are fully SMS compatible.)
Much the same case with the Saturn. The 68000 included is primarily utilized for audio, however, it could be used for running general-purpose code as well. Sadly the schematics I have access to do not include the 68000 so I am unable to analyze exactly how it is wired to the rest of the audio subsystem. I doubt it is directly connected and simply writes to it via the bus arbitration logic, so any of the system's processors could be made to push audio instructions.
That said, nearly all Saturn games perform game logic processing and (where relevant) 3D transformation calculations on the Master and Slave SH-2 processors. A comparatively minimal workload is put on the 68000, so overclocking this processor will have an extremely minor, or nonexistant, impact on game performance. What makes a system 'overclocked' is debatable, but I would argue against selling a machine without its primary CPU or Video logic performing more optimally than it did before the modification was performed, (which an overclocked 68000 won't produce) would be deceptive, since the user will experience no performance benefits for their money.
Compared to, say, a system wherein the CPUs run 30% faster and game slowdown is reduced, it'd be hard to justify spending an extra cent for an overclocked 68000 in the Saturn. That'd be like paying more for a high-end gaming PC wherein only the sound card's processor is overclocked, and overall game framerates remain wholly unchanged.
Edit: By the way, overclocking the 68000 shouldn't speed up the music tempo unless it was also recieving instructions at a rate that permitted it and there was no secondary means of synchronization. Overclocking the Z80 in the MD/Genesis causes audio to speed up, seemingly because the 68000 is writing audio instructions to the Z80 as fast as it can receive them, and there is no means of timing them at the 68K's end since the Z80's speed is (ordinarily) fixed at ~3.58 MHz so its # of operations completed in a certain amount of time is uniform.