And Mentzer was also a bodybuilder where balance and symmetry was crucial. Iso exercises help specifically target and bring up the size of muscles where your genetics may have shorted you. As I mentioned above iso exercises are needed for looks, not so much for anything else.
In terms of pure functionality, there is very little need for isolation work. Compound movement execution can usually be altered to some degree to put more emphasis on lagging areas but at the end of the day, the weak link in the chain is ALWAYS going to get worked when doing compound movements.