Ok, it's not very nice on the sole of your shoe, but it's generally biodegradable, and I have eyes not to step in it (particularly at the beginning of a walk where it's inevitably at its worst).
True, it's not exactly hygienic (but then neither is bird mess - it's just smaller).
So why do people "clean up" after their dogs, and consider it better to leave the bags in the undergrowth? This way it doesn't degrade AND it litters the place! AND there's just soooo much of it around certain local parks.
Bah!
(I know, I'm preaching to the converted, but i'd rather see dog mess than the dog mess & plastic bags).