Personally I prefer to see games displayed in a square pixel aspect even if it means letterboxing, and I believe that's how they were intended to be displayed, since Europe and our PAL standard was the big market, and a square aspect is just easier to work with.
The Amiga hardware has no fixed heights in its display modes and can display any number of scanlines up to the limits of the format. My brother's PAL TV automatically switches to 4:3 when it gets a 60 Hz signal, and in this mode it shows a total of 224 scanlines, but I think a real NTSC display will show the full 240 lines (anyone who knows?).
I think the main reason many Amiga developers went with the short 320x200 (even if they could have used say 320x240) was because to them it was the established resolution that could be viewed by the biggest audience --> more sales.