Never felt the need for a Sog, depends how you use your loo. If you want to open the valve and plop straight into the tank then it might help. I'd rather plop into a bit of water, then close the lid and activate the valve, No smells at all, maybe mine doesn't smell.
Well after that, I really must get one!SOG is great to let the whole world that your using the loo! You flush, and when the pump is running, it turns on a fan, thus pulls air from inside the motorhome, down trough the toilet, down into the poo tank, across the wee and poo, and pumps it outside, over the next doors dinner table (if your loo is on the drivers side) and out under you awning if you loo is on the caravans door side!
You take a perfectly good Thetford toilet, drill holes into it, bodge into the wiring and cut a hole in the door, just to stink the high heavens out when you empty it, and stink out your awning or next doors dinner every time you have a wee (do use if your easyily embarrassed lol) oh yes and the simple slide in action of the cassette, is compromised as you have to hook up and disconnect the fan to the tank every "in and out" action.
Formaldehyde by the way can be used in chemical treatment plants, we didn't think it could be when we built our camp site and had special tanks installed for chemical toilets. Only to find that the problem is the high concentration of solids in cassettes and holding tanks that can cause treatment plants problems rather than the formaldehyde.
Of course none of that is a problem with a SOG as you never get chance to build up solids as the smell is so bad you have to empty it every day
I guess that's why Thetford don't add it as standard!