http://www.spookhouse.net/angelynx/comics/kusarigama.jpg
Not the most realistic picture, but let's go with it. Big scythes are impractical, but kamas, sickles, etc. have been used effectively in battle. It's interesting, both the East and the West have used scythes as a weapon (well, sorta). In Japan, in fact, it was a fighting style (http://en.wikipedia.org/wiki/Kusarigama). So scythes can be used effectively as a weapon, just not in the way that they're usually associated with.
As for why they're associated with fighting...look at technology and the media. Scythes are no longer needed to harvest, given our technological advances (besides, it was tiring and wasted a lot of grain). And then,look at shows (specifically anime and Japanese games), and see where the influence comes from. When people watch stupid stuff happening in fiction, most of the time they'll duplicate it in their works of fiction (or in real life). That's why such an impractical weapon is associated with fighting.
Oh, and as for improvised weapons...I don't know about in the West, but in Japan they banned both Okinawa and Taiwan from having weapons (I think it was both of them), so they came up with new ones: nun-chucks, tonfa, bo-staff, and the oar (which is kinda badass -_-). I think tonfa were the threshers actually, but I'm not sure.