KevinL: My interpretation is that while the recommendation is that children under the height of 148cm should be in a child restraint (and children under the age of 5 must be in a restraint); if they are between 5 and 8 they only need to be in a restraint if there is one available in that vehicle.
I've been wondering about this with adults, I know a couple under 148cm who could do with a boost, the reasons for kids should still apply.