Why bother with such trivial changes, and what is the benefit of it? For all I know hotspot can remove these so I doubt there is any performance issue.

Certainly, in most cases, if the JIT can't optimize to a cpu register,
it will reintroduce the temporary anyway.
That is true even if you don't use a temporary in your code.

The optimization to a register (rather than a stack entry)
will be done regardless of whether the temporary is explicitly declared.

However, with most modern cpus it makes no real difference
since the memory (for smallish methods) will be cached on the cpu.
Making it nearly as fast as registers anyway.

I'd imagine that hotspot removes unncessary cases if it is JIT compiled?
If it is not JIT compiled then it doesn't need optimization.

But the unncessary cast does make the code harder to read.

In general only make these changes if it improves the
readability/maintainability of the code.

e.g. removing dead/unnecessary code gives less to maintain
in future.

Removing temps doesn't change the semantics of the code
so it doesn't really change anything.
In some cases, temps are more optimal.

e.g. code that doesn't use a temp that should:

doSomething(getSomethingLotsOfWork());
doSomethingElse(getSomethingLotsOfWork());

object.getSomething().getSomethingElse();

temp = object.getSomething();
temp.getSomethingElse();