My understanding of the rationale was that it related to halftracks generally having firing ports for small arms. As well as giving a covered firing position, those would tend to provide the support you mentioned. Add it all up and you get that, overall, shooting at people from a stationary halftrack was about as difficult as shooting from a grounded position.
Now, for Mounted Fire from other than an armored halftrack, the first point is that ASL treats unarmored Passengers the same whether they're in seats or a truck bed. Firing from a seat is awkward. And the truck bed may be enclosed in a canvas cover, rather than open to the sky, horribly limiting their visibility.
And of course, if it's Riders doing the Mounted Fire, they're trying to stay on the vehicle and probably firing with little or no support, as well as a bunch of other problems.
John
Just to clarify, only the SdKfz 251 Ausf A had any sort of ports in the passenger compartment. The Ausf A was produced in small numbers and replaced by the Ausf B which was somewhat simplified by, amongst other things, no (openable) vision ports except in the driving compartment. None of the later SdKfz 250 or 251, US M3 SC or M2/3 h/t had any sort of vision or firing port in their rear compartment.
The sides of the US M2/3 h/t track barely reached the top of a seated passenger's head and with the SdKfz 250/251 not even quite that. The US M3 SC's armour top edge sloped down a bit towards the rear. All fire, whether the crew/squad/HS or vehicular weapons would have been over the top of the vehicle. Direct fire weapons on h/t variants nearly always had addition shields to address this issue as had the front MG on SdKfz 250/251s.
The purpose of the various APCs in WW2 was to bring the troops through the shell and MG/rifle fire and allow them to dismount very close to their objective. How close depended upon national army doctrine, leaders, experience and particular tactical situation, but they were battle taxis, not assault vehicles. Sometimes they stayed around to give covering MG fire, sometimes not, again depending upon nation, etc. Often the troops were dismounted behind cover as the APCs were just too vulnerable and valuable.
But this is ASL! The fact that only maybe 1 in 10 of bullets comes through the side is likely less distracting than all. You have the equivalent of a moving wall/ASL 'wooden' building for protection. This often results in ahistorical h/t tactics in ASL, but h/t usage is not unique in that respect (see VBM sleaze, etc). The thing about ASL is that that
you choose their usage. They are very valuable as protected (proof against squad inherent FP) transport. Use them as assault death stars and you too often will loose them, but that may, in some cases, be worth it. Others have pointed out various other tricks, However you use them, remember that they are transport and moving a reinforcement/reserve or running or exploitation force through a defensive hole may be more important than a mobile MG nest equivalent.
One aspect that ASL totally understates is ongoing OBA during a battle. In WW1 and WW2 artillery was, by a large margin, the main killer. Barring an unlucky direct or very close hit or airburst, the various APCs protected their troops against artillery. In WW2 that was at least as important, if not much more so, than protection against rifle/MG fire. Defensive artillery fire was the norm from even well before WW1, but is rarely seen in ASL. To represent this you would need to subject non AM moving attacking infantry to a 1FP residual fire attack in a band 3 or 4 hexes wide about a similar distance out from a defensive line. Because that is ignored in ASL the protected transport nature of h/t is a bit understated, IE getting troops through the "beaten zone".