There are two camps here:
There's the "I prefer the original pixels" camp, which demands a crisp image at all costs. This camp can only be served by showing the original 160x144 image centered on the screen, because non-blurred upscaled 320x288 pixels doesn't fit in 320x240.
And then there's the "If it's going to be blurry, might as well use all
the screen space offered by this console" camp, which is served very well by the full-screen scaler.
The first camp is going to be like "Why resize it at all
, and get blur?". The second camp is going to be like "Why resize it only this little bit
and waste the rest of the screen?". As a result, it'll be an option value that just requires another button press to skip, for everyone.
There are also many models of GB, GBC and GBA; should options be added for each model, just in case someone's preferred model is not the same?edit:
There are also magnifier add-ons that people used on their GB, GBC and GBA. Each of those has a different magnification factor, too, which could be yet another option! But you can simply pretend that the full-screen modes in GCW emulators allow you to get the true experience of using a magnifier add-on, without the fuss of an actual magnifier add-on
In short: This is pointless.