They used to be synned, but then slimming world decided to make them free, as you are unlikely to over consume them and they are really just used for flavouring. I can't imagine anyone eating just a stock cube. lol.
I don't use them in all my food, as they are quite high in salt content and my 4 year old eats pretty much everything I do. I buy the reduced sodium ones and tend to use half a cube in things that don't need much and then in soups/stew etc where plenty of water is added I probably use about 1 cube, as I find the vegetables add their own flavour, so you really dont need much.