But, the traditional "-2 to +3" is not very good way to express this, as -2 usually results in dark, muddy images when some of the important parts of picture fall on the toe, whereas +3 can be quite good because of the long linear curve.
Of course, a good system to rate the EI of film is such that it takes all of the film's speed into use, as it is usually more practical to shoot with as fast film as possible. So, marketing a film which could be shot at 1600 as a "400 film" wouldn't be wise for anyone, not the manufacturer nor the buyer. This is why there is not so much "hidden" range in the lower part of the curve --- exposure is measured by the ISO standard and by most film users as low as possible to still get good results; thus, underexposure starts eating away the image quality quite quickly, bit by bit. Overexposure is different, though! This is why single-use cameras usually use 800 speed film.
Accidental two-stop underexposures can usually be used, but they can be disappointments. OTOH, accidental two-stop overexposures seldom are disappointments, unless there is some scanning issue involved.