Flexible couplers absorb the minor vibrations that come with
hull flexing and mount flexing. They can pick up small vibrations from the prop as well, deadening them before that can be transmitted to the boat.They can reduce felt vibrations when the
engine is chugging along. They are not a remedy to not aligning the engine and the shaft properly in the first place, or worn out engine mounts. So, yeah. I use both a flexible mount and a soft (relatively speaking) thrust plate between the engine and the tranny. I don't know of a down side if you did the alignment properly.