writes:

If visibility is 4.5 miles, you are by defininition in IMC.

Are you always solely on instruments, or is there any reason to be
on instruments at all, with a visibility of 4.5 miles?

If you're in IMC, you need to be on instruments. At 4.5 miles, you're only
60-90 seconds away from not being able to see anything. Are you ready for
that?

Hard experience has taught the aviation industry that just being clear of
low-visibility weather isn't enough; you have to keep a safety margin between
you and that weather. That means that you have to be on instruments once
you're inside that margin. If you wait until you really can't see anything,
it might be too late, especially if you are unprepared.

Are you always solely on instruments, or is there any reason to be
on instruments at all, with a visibility of 2.5 miles?

If you're in IMC, you need to be on instruments. In IMC, instruments become
your final reference, no matter what you see out the window. When visibility
drops, instruments are your exclusive reference, no matter what you feel.