I wouldn't use the caddy to carry a plasma cannon, and it's generally not for weapons (except melta bombs, possibly grenades and maybe disposable one-shot launchers). It's for tools- camoleoline tarps for building a hidden camp, or climbing gear for scaling the exterior of a hive spire, for example. Similarly, I wouldn't take a servitor or a serf into the field- I'd be dragging the thing myself; having a servitor or serf stomping around in the brush would be tactically unsound.
I wouldn't let someone take additional primary weapons with it were I gming- as previously mentioned, those are ancient arms and sacred relics. I would, however, allow someone to fill his little red wagon with melta bombs, promethium and trip wires, and use it to booby trap the area approaching a hide, where he might camp out with, say, a stalker and a heavy bolter. So the kill team goes in and does whatever, then they coming running back to the extraction zone with their hidden Brother up on a hill picking off leaders. As the kill-team gets closer to the booby-trapped area, he voxes guidance through the the dangerous area. Their pursuers get no such guidance, and start tripping over 40k equivalent claymores.
In essence, it's sacrificing the requisition you might use for heavier arms and armour for a versatile selection of tools- that you may have to abandon, because a little red wagon can't go everywhere, and it's better suited to support than infiltration. I think that's wholly in the spirit of requisition: determining your role in the coming mission.
Re: Codex:Space Marines infantry model with more than one basic or heavy weapon: terminators with missile packs and Storm Bolters.
As a side note, weren't you arguing in another thread about how GW Crunch Marines fall short of GW Fluff Marines? I could easily see a bolter maglocked to the side of a heavy bolter ammo pack.
Unrelatedly, you are now picturing a Radio Flyer stacked chest high with meltabombs and missiles bouncing down a hill at a Hive Tyrant.