I served with another nation for whose military it was perfectly normal to adopt local acting unpaid rank for many functions.
Well, it might work...until something goes wrong. I present the case of Brig Saunders, assassinated in Athens in 1999. Local, unpaid rank and his widow (quite rightly) argued for some years that she should receive a Brigadier's Pension on the basis that he was wearing the rank and doing the job when Nov 17 slotted him. I'm sure there are quite a few examples from AFG and Iraq where 'local' rank ended in a few tears. My replacement in AFG was Army and given acting rank because of parity with the US - it took quite a battle to persuade MS that this was necessary.
I certainly think that consideration should be given to putting a ceiling on paid senior ranks within the UK military.
There is. But as Jim pointed out, the total numbers are miniscule. However, if someone is doing the job of that rank, what's wrong with paying them for it? (qv my earlier comments about Brig Saunders).