It is not right to say that it is always dangerous to use long cables, or that shortest possible cables are always constitute the best alternative in electrical circuits. However, shorter cables do present some advantages. Some of these have been identified in the answer above. I would like to add the following to the ones identified.
- Longer the cable, greater the chances of their getting damaged. This may result in leaking of current due to poor insulation. For high voltage circuits this can be dangerous.
- Longer cables cost more.
However, in some cases it may be desirable to increase the length of cable to organize the neat cable layout that avoids excessive crisscrossing of cables, and cables running across some of the other components of the circuits. In permanent electrical circuits running over long distances, provision is usually made for some additional length of cable so that any subsequent repairing of faults in the cable will not result in the available length of cable falling short of the minimum required to cover the distance between the two ends of the cable.
In a circuit the primary purpose of using cables is to allow current to flow from one point to another. It is therefore best to use cables of a length equal to the distance between the points that you are trying to connect. This will reduce an increase in resistance caused due to longer cables. It will also reduce the mess that could result from long cables getting entangled.
Also, when using long cables some may get disconnected from the points of attachment when the other cables are moved. This will lead to ends that can potentially lead to shocks. Therefore keep your circuits looking neat and working efficiently with the use of cables of the right length.