Very good point and one that I questioned myself. I had no idea that the mount had split until I removed it. I guess that as it hadn't split all the way through and with the poly stabilisers being as stiff as they are stopped it from getting any worse. I'll see how it goes.
You can remove the mount by supporting the engine and lowering the subframe so it's not the end of the world if it needs to be done in future.
If you've got poly bushes elsewhere then even more reason to replace it with a fresh mount.
With the other mounts being stiffer you'll find that the engine movement will be loading that mount up more. Having the stiffer mounts won't stop the engines natural movement, but if the stiff mounts can't flex the energy/movement will hit the weaker ones.
Rubbish explanation but hope it kind of makes sense.