Well, this is one of those instances where a little bit of knowledge can make you think the product is great!
Good grounding practices make you tie grounds to a common point, in certain circumstances. Also the shorter the ground path, the better. But....
This also depends on the current, voltage and frequency of the energy in question.
When you're at 40-60K volts, and low current, such as in ignition systems, resistive losses, unless grossly severe, will not have a significant impact on the performance. So using the metal of the engine as the ground path is very acceptable.
Since this is also an impulse type of waveform, inductance of the "signal" path can also affect the rise time of the discharge, however, I'm not sure that this significantly affects the spark discharge at the plug and the resulting fuel burn. (Impulse signals contain a broad range of frequency components and the faster and higer the voltage and risetime, the higher in frequency the energy is.)
However, it does affect the radiated energy and resulting interference to radio recievers, which is why resistive ignition leads are used to dampen this.
For high RPM engines, a single ignition coil for each plug, allows a longer charge and recovery time for the ignition coil, and this can make a difference. Also, mounting it close to the plug, reduces energy losses as the path length, inductance and resistance is reduced.
I'm no automotive expert, but that's my understanding based on my electronics and physics knowledge.
Over time, the leads can degrade. If I recall correctly (and someone please correct me on this, if it's wrong...) many ignition leads are carbon based instead of copper wire, to keep the noise down. If this develops many cracks and gaps over time, the resistance will become significant and degrade performance. So you'd want to change them.
Someone mentioned sheilded wires, where the ground was carried on the outside. Where I see an advantage here, is in noise suppression, not so much the return path. If the core of the wire is copper, then it's going to be a lower resistance and better ignition energy path. But as mentioned before, it's going to be noisy in the RF (Radio Frequency) area and cause interference to radio receivers. Sheilding with a grounded sheath, could reduce this radiation.
The downside to this, is the insulator between the two has to withstand the high voltages over a number of years through humidity and temperature extremes without breaking down electrically. So the wires would fail faster and cost a lot more. Again, I don't think the performance difference would be significant. Especially with a computer that can dynamically adjust ignition timing to maximise performance.
It would be really interesting to hear what an Automotive design professional with expertise in this area would have to say...