Some bikes need the tubes to keep the chain from rubbing on solid parts--not all bikes have clear chainlines.

I bought a first bent (a SWB) that had a lot of tubing, it was bad for two reasons--it had DRAG on the chain that you could feel. And secondly, it made cleaning the chain well just about impossible, because the tubes were always full of gunk.
This is a page showing how I cut mine down:
http://www.norcom2000.com/users/dcim...ster_tube.html
on this bike, the tension-side was clear, but the return side was not--so all the tubing could not be eliminated.

Also we note: in the US, hardware stores sell plastic piping that is pretty much identical to the tubing that comes with bikes--except that the hardware store PTFE pipe is often a different color--usually milky-white. It is the same size and texture however, and is pretty cheap to buy per-foot. It is often curled a bit because it is shipped on rolls, but it does relax and straighten out if you heat it up a bit with a heat gun.
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