As with clothing, layering is a good idea.
At its most simplest, would a thief rather attempt to steal:
a. A bike with no visible additional security
b. A bike with one item of visible security
c. A bike with more than one item of visible security
Yes, there will be examples of bikes (a) which are never stolen, just as there will be examples of bikes (b) and even (c) which are stolen. That is nothing more than the pound of butter in the fridge rule in action.
Setting aside the likelihood of a thief’s preferences, it is though easy to get carried away, A diamond rated chain is, undoubtably, strong. But, if the bike is not chained to something immovable, then it is simply chained to itself. This will not deter a thief (with or without an angle grinder) who simply wants to lift the bike into a van and be gone. Set against this, a lot of thefts are simple ‘push aways’ (ie no van involved and sometimes no angle grinder either *) where a chain might well help as a deterrent, if nothing more.
The best advice? Do what YOU think is necessary for YOUR bike. If that is nothing, great. If that is layers, great also. Something between? Great, as well. But, whichever you chose, do NOT ignore the butter in the fridge.
* Thefts from bods’ garages or off their drives, are a classic example.