Perfectly smooth motion paths such as splines/arcs/curves create or suggest tension and coordination, because you need tension and coordination to sustain such equal movement for more than... maybe a second, maybe less (so it's also a matter of timing, in that the eveness of the motion = even timing, but that's for another post ;) ), you need tension and coordination in order to be able to draw a circle (unless you draw it fast and you've drawn billions of circles before, aaaaand you can normally never really predict the outcome). So it looks more like your character wants to be arteeeest, and tries to describe an arc, rather than grab an object. It can look great actually, and cartoons do it all the time... But natural motion has normally more random curves (most curves, not all) in a continuous slow/fast/slow/fast. Relaxed movement especially can't be too smooth (it might have to be soft or almost-smooth, but not computerish-smooth).
Edit:
But! There still are lots of movements where we have arched spacing like the second line. After much deliberation... well, OK, right now I think even timing creates tension, not round spacing. Actually, round spacing, broad arcs, look great and should be used in animation. Realistic movement might not be as nicely arched, not as round, but it still is arched. It's got more turbulent spacing sometimes, but it always flows. I should write something on cartoon vs realistic flow... basically, cartoons have more exaggerated movement, and so they need more rounded arcs, and often they have to be animated along a line of action, in order to read and flow well. That's not something that would happen too much in reality (happens with fish and other creatures that flow along their line of action, but rarely with humans), but it feels natural anyway, in cartoons, and helps the flow.