Not meaning to play devils advocate but I agree with Atari...
Although C. irritans is one of the most common problems with new fish, it is not the only parasitic infestation you will come across. If treating new arrivals with hyposalinity and it developes an alternate problem such as Brooklynella or Amyloodinium, the hypo will be useless. There are certain meds which can easily be used in conjunction with hypo but copper and formalin cannot. In order for the proper treatment to begin, the salinity in the tank needs to be raised first. It cannot simpley be elevated as quickly as it was lowered. It can take about 3-4 days to bring up so as not to shock the fish. Any sooner and it could (but not always) finish the fish off.
The time you take to switch treatments gives more time for the parasite to do it's damage.
Cheers
Steve