For me incendiary rounds are a regular secondary ammo choice for carronades and weapons with arming time. I see how one can like incendiary on gats, but I definitly prefer it beeing fired with greased rounds (preferably by a buff engie) any time. I think it useful on flamers, but it isn't my first choice and as I barely ever put a gunner on a flamer, seeing flamers being shot with incendiary rounds will be a rare spot on my ships.
A way to make incendiary rounds more attractive might be to scale its ignition change or number of charges inflicted with the weapons shots/time ratio. This effect would have to be capped somehow though, as beacon like fires set by heavy flaks or mercs would probably ruin the balance.
Also think it shouldn't be the ammo type with the highest projectile velocity reduction.
A question regarding incendiary rounds:
does the chance of ignition apply to the primary damage, the secondary one or both?