Mostly the extra fuel line is for when you are on your lid waiting to get marshalled.
With people either running a clunk style tank (Losi and Hot Bodies) or modding a tank to have a clunk, the extra line that people ran isn't needed as much. If you want a little more run time, add in a couple of fuel filters (with the guts removed) to gain some extra fuel. Depending on the filter you can gain 3 to 5 cc's. For club racing you could get away with that, but at larger events adding in the extra filters or fuel line could put you over the max. fuel limit.
If you really feel the need to do either of the above, be sure to check the length of the exhaust line.
My theory is that if you add in a bunch of fuel line or filters, you may need to shorten the exhaust line to compensate for the added fuel line or filters to keep the system in balance. If you run a short fuel line, adding some length to the exhaust line will help to balance the system.
As stated in the first responce, there are many theories and mine is but one. I'm sure that there are plenty of people that could better explain the effects of longer/shorter fuel/exhaust lines.