Would you advocate using them on lighting circuits as well Christian,
or just on the circuits which you would otherwise put on the RCD side
of a split load CU?

Not on lighting circuits.

The only justification I could see to put them on lighting circuits is on a
TT earth, so you don't need an overall RCD (either a time delay or a
standard one just for non-RCBOs). However, even then it would be a very
expensive option.

Christian.