It really makes no difference if you go direct battery or your aftermarket sub-panel, as long as the power is clean and interference-free.
I think the recommendation stems from two things. The first is interference, minimal junctions and balanced power helps to remain free of common mode noise. The second I suspect is practical, to guarantee the return is sufficient for the power. Vehicle accessories are often simply grounded to the nearest chassis tie point without much regard for the fact that even relatively low power 50W mobiles are consuming 8A to 10A of current. A 100W HF will consume beyond 20A on peaks.
A #8 screw with a star washer and 16 AWG to the body is insufficient. Not only might it be a fusing hazard it can easily cause problematic voltage dips. If the ground has even 0.25 ohm of impedance that means at full power the voltage will dip 2V from the ground elevating. Even if you do ground with a big enough wire, often bodies are isolated with rubber pucks or body seams are not well bonded electrically, so the common denominator must be to tell everyone to run to the battery to cover all possibilities.
Battery power at ~12V is already something of a compromise since most radios are designed to actually run at 13.8V and will only really dissipate their full rated power with the engine running. If you're sitting with the engine off and the battery at 12.3V it's doesn't take much ground impedance for the radio power to compress to 10V or less, which means you're probably going to be transmitting with a lot less power than you think, if the radio doesn't blink off in extreme cases.
So you are 100% correct that there's nothing magical about full runs, but it's prudent to make sure if you decide to deviate from the recommendation that you account for all the variables.
The truth is a car body and frame are excellent conductors but you have to make sure there are jumpers equally good to make sure they are all tied together and back to the source. That's another critical thing missed on most cars, the final jump to the battery isn't necessarily going to be very good between the body and negative terminal. The largest connection from the factory is between the battery and engine block for the starter. So it's not unusual for there only to be a small wire connecting the body to return, which you might need to augment.
