You won’t see it in QT, you’ll see it when you composite the QT over other images/video, especially if you used a colored “Sky” background instead of the all black “Premul” option, the latter having only a slight uniform gray halo.
Multipass OpenExr files are Blender’s native rendering format. All images imported into Blender are converted on the fly to RGBA float, even if the input image doesn’t contain an alpha channel - such as .jpg format. This makes Blender unique in the world of compositing software except for Adobe AfterEffects which also natively assigns an alpha to all input images (but does not convert them to 32bits per channel float like Blender).
OpenExr format is capable of containing an infinite number of arbitrarially assigned channels such as Normals and Vector (available from Blender) or camera tracking data (not yet available from Blender). This amazing degree of flexibility has made it THE industry standard for digital imaging. 32bits per channel however seems to be a bit of overkill since it encompasses just about every shade of color that human eyes can process and a whole lot more. The precision level is amazing too, to the point of only failing in an exceptionally minor academic sense though your eyes will never be able to differentiate such minute flaws.
While 32bpc is overkill for most work, standard Openexr format in Blender (not to be confused with MultilayerOpenExr files) include an option for Half type or 16bits per channel files. These allow extreme levels adjustments without introducing banding artifacts, are more than adequate for compositing, and save an incredible ammount of disk space as opposed to their MASSIVE 32bpc cousins.