Well okay, that's a little harsh to say, but it's not exactly untrue. Let's get to the point.
There's a number of models who are not integrated in the general formation OOBs. These units can be integrated through the use of battlegroups, but that's inconvenient and labour intensive. You basically have to handcraft every single battlegroup.
That can work for the construction of a small number of formations for best effect of precious resources, but having to do that for an entire army is... annoying. Using the OOBs is much more convenient.
The useless models are the Mechanized Quad Machinegun, Mechanized Artillery, AT Gun, Assault Gun, Walker, Heavy Walker, Recon Buggy and Motorbike Infantry. All of these can fill roles already in use in the various formation OOBs described in the game.
The Mechanized Quad Machinegun and Mechanized Artillery, for example, can easily replace Machinegun and Artillery units in motorized and mechanized formations. They would also then not require a vehicle to carry or tow them around, which saves a bit on resources.
AT Guns offer substantially more hard attack and defense than RPGs, and could work as an alternative there. Assault Guns and its bigger brother the Tank Destroyer could be an option for this role as well in mechanized or motorized battalions.
The Walker and Heavy Walker are mountain capable equivalents to the Light and Medium Tank models, which on some maps would be quite useful. Dislodging an enemy formation from a mountain you can't outflank is rather painful without vehicle support. Or even with vehicle support, if you've got a road you can use, but the Walker and Heavy Walker do away with those requirements.
Early game, the Recon Buggy is a fast moving raiding and support vehicle, which could be paired with a motorized infantry unit or with Motorbike Infantry for a punchy rapid response unit. Sure, that formation would definitely be replaced over time as effective counters to the lightly armoured buggies become more available and front lines more defined, but Motorbike Infantry would remain an effective option for Mechanized and Motorized forces. An Armour Infantry formation could easily be made up of tanks with motorcycle infantry for escorts, instead of trucks hauling around infantry.
I am deliberately leaving aside Jetpack Infantry, Monitor Tanks, Shield vehicles, the various very long range artillery options and the Automated MG and Turret, the last two of which probably should have a dedicated automated OOB combining them with Robotic Infantry and equivalent to Grenadier Infantry. They're just excellent for creating a stationary defensive line.
I'm leaving them aside mostly because I've no experience with the Jetpack Infantry, Monitor Tank and Shield Vehicles, while the very long range artillery options would not get involved in anything other than an artillery duel unless something has gone horribly wrong or an aircraft focused expansion has arrived, and as such do not require mixing with other models at this time.