Maybe I misunderstand the situation you are describing, but if you are handling an IRS FAULT light you might have a failure of that IRS in NAV or ATT. If only the NAV mode has failed you lose groundspeed and present position data from that IRS, in that case switching that IRS to ATT turns off the FAULT light and leaves the other IRS intact and supplying the FMC with position information. You lose nothing by following the procedure, just switching to the other leaves you with only one normal attitude source when the other one might still be working.