Roman Army

[Ludens]

There is of course the option that they were held in reserve behind the reliable troops and used to exploit weak points without messing up the 3 line formation.

Possible, but the way you depict them, they armed with sling and knife. Not exactly the kind of troops you can use to exploit weak points. I agree that it is unlikely they had to cover the triarii's retreat, but they are basically armed as skirmishers (and poor skirmishers at that), and yet that wasn't their function. So from this I would conclude that they probably didn't fight at all, except as a last resort.