When I'm flying an Odyssey, I do use the auto-fire plasma cannons, but also manually turn them off when needed due to flux concerns. Being able to plasma burn, vent, and bring up shields where I need them (often in a port-stern arc) is quite valuable if I'm trying to solo a late game bounty fleet.
The type of ship which I think does benefit from Omni-shield hull mod conversion is the Onslaught (or mod ships like it). High armor, somewhat broadside capable, and really slow to turn. The fact the shield is smaller and is more willing to be dropped helps, as the ship is utilizing it's armor more, while still seeming to prioritize high HE projectiles like reapers, and certainly improves it's outlook when it's isolated from allies.