Hyposalinity is bascially a treatment for ich. It is non-medicating, thus it has no ill-side effects. It is safe for fish, and even helpful to sick fish as the lower salinity results in the fish not having to expend as much energy in regulating osmotic pressure, thus, that extra energy can be used by fish to heal.
As for using hypo generally for new incoming fish [in the absense of ich] this is a matter of preference. We have been suggesting it here because the treatment seems to have some positive results in treating other types of [non-ich] parasitic problems and, as mentioned, the osmotic pressure is more equalizied in hypo conditions, thus reducing fish stress.