To refer back to the earlier posts on poly-phase systems: a major advantage of these is that for a balanced system, there is no neutral return current. For example: 3 single phase AC circuits would need 6 conductors; the same result can be achieved with 3 conductors if connected 3 phase. In practice, the 3 phases would not balance perfectly so there would be a residual neutral current but it can be of reduced section (it is also possible to use a 3 wire system with no neutral). But wait there's more...because there is no neutral current, the total volt drop across the circuit is halved for each phase, allowing further economies in conductor size.
This great idea was invented by Tesla in the late 1880s and works so well that it has not changed in principle since. And I think these systems still will be around in another 100 or 200 years time.