i think it has to do mostly with the font. for example i have a circle a (duh) and besides not wanting to scream 'custom bike' i think their logos aren't too good looking, but the head badge rocks.

on the other hand, i have 2 other bikes with decals which i think would look bad without their decals even though they are deteriorating (one is road the other track). maybe i like the fonts, maybe it's the 2 tone paint job, or maybe late 80's italian bikes were just made with style and age well. a lot of american brands seem to have crappy logos.