Problem is, put too many or too good weapons on your APCs and you encourage it to stick around and fight. If it IS designed to fight in direct support of the troops, then it is a ISV/IFV and a different kettle of fish in training, deployment, and tactics.
To an extent: Agreed. This really comes up to doctrines-aka what your infantry forces are
for, and it also smooshes into the mix of whether you're going to use ALL the options or not. "Mechanized" infantry (by the rules) fits better the thin-skinned apc concept, separate dismounts fits better with IFV's, and the canon APC models?
That's just really shoddy IFV units (has guns, not much use in support).
The rules kind of set up for two concepts pulling away from each other-on the one hand, you've got separate infantry and vehicles, which lets you have much better vehicles with much better mobility, but sticks you with leg, or at most, motorcycle troops. On the opposing hand, you've got "Infantry" that pull their half-tracks up to the second floor somehow, and those tracks aren't
much better than their plate-carriers and helmets...except that they're slightly faster than walking (only just) because they've been abstracted that far.
the two concepts really don't mesh logically, but both are in the rules, and even show up on the same table.
sometimes at the same time.
Conceptually, I'd say how you organize your forces really should boil down to what you think your doctrine OUGHT to look like. (and keep in mind, not everyone will agree with your ideas, or mine, or someone else's, but as long as it's rules-legal you've got a LOT of options.)