Why is it wrong to have multiple ground straps between the chassis and engine?

There are multiple ground paths between the engine and chassis (mounts, throttle cable, piping, shifter linkage, etc.). Why is it wrong to have multiple reliable ground straps in addition to the unreliable ones?

We want to limit the paths our current can flow along, so we can have a better idea of where things are flowing in case there are any issues to troubleshoot. There isn't much you can do about some of the mechanical connections between the motor and chassis, but you'll find a surprising amount of them are isolated by rubber sections along their length, to allow the motor to move in relation to the chassis.

So having a single ground strap is done for operational reasons and not electrical performance?

Essentially, yes. Although it should be sized, crimped, and connected such that it has excellent electrical performance :-).

What about causing ground loop potential that introduce undesirable hum, noise and interference when there are multiple ground conductors (paths)?

That is basically what Zac was referring to in the first sentence of the first post, as the current paths merge and part the voltage potentials will vary very slightly which can cause issues with some electronics. Where practical, it's generally a good practice to use the same power and grounds for circuits that relate to each other - however, in some applications, smoothing circuits (or even simple capacitors) may also be required.