The purpose of bus wire around the layout is to distribute power to the track. To rely on the track alone to provide a good connection is a gamble because, as Jim points out, there are unreliable track connectors, pins, and other structural reasons to doubt the integrity of the circuit. The bus wire and the drop wires connected to the track every few feet ensures that the power gets distributed more evenly to all sections of the layout.
To reconnect the bus wires to the transformer after going around in a loop only affects the path that the power takes to get to where it is needed (electricity takes the shortest path). It does this on its own and is not a real concern for the bus wire distribution technique.
