I had to look them up...
11.4 Litre Dinghy Mate - Boatnbits
I can't work out if it's a brilliant idea or one looking for a problem that doesn't exist.
But thinking about it I wouldn't use one with any of the under 4m SIBs I've owned as weight distribution is important and weight all at the transom is not usually where you want it. I use a 20l tank which is usually half full to full and makes excellent moveable ballast. When one up my tank is always well forward.
It might suit a setup like Gurnard's though where he has a larger SIB with a lot of kit up front that would balance out. I could also see it suiting a larger displacement speed grp boat.
My only other thought is does the tank hang on those two brackets at the top... if so I could see potential for them breaking from the tank with extended use in bumpy water.