From a realistic stand point, do these guys have any tactical value in the field?

4665048
Well Grunts use methane, the Flood is VERY vulnerable to fire, and flamethrowers are still great at dealing with entrenched positions when you can't just bomb them.

Fire is always a useful tactical weapon.

4665048 They can burn out hidden enemies.

4665048 Well what can they do other than burn stuff?

4665600

They can cook up a mean potato stew.

4665048 FEARMONGER THE SHIT OUT OF THEM!!! MAIM KILL BURN! MAIM KILL BURN!

In game the flamethrower units were useless

If you knew what they were actually meant for then you'd know they are a biological safety protocol weaponized. So strategic value? When facing entrenched enemies in close combat.