"BUT IF I HAVE AN IRS WITH EXCESSIVE DRIFT WITH NO EADI/ADI OR ND FLAGS SHOWING I AM DONE WITH REVENUE OPS. SEEMS EXTREME TO ME."
I suppose one could ask... How excessive? If an IRS is drifting wildly, it may think it is in a completely different place to where you really are... and a different magnetic variation might apply to this place, rendering the onside HSI/ND heading indications inaccurate (unless you have switchable IRS sources for instrumentation). If you selected ATT, then you would have to continually enter the correct magnetic heading in the CDU.
The risk may be greater than this, however. Can you say that the cause of the IRS drift is not related to an IRU internal problem which may affect the ATTitude components of the IRU later on in flight?.... You may lose attitude as well as position info (Losing ADI/PFD data, Radar stabilization, Autopilot redundancy, etc).
Just because you don't have any flags on departure, it doesn't mean you won't have any flags later on in the flight if the drift becomes excessive. In particular, I wouldn't be planning any long overwater/navaid-free IFR trips. Remember, things like FMC's are only as good as the info which goes into them.
My 2c's worth
Regards.
Q.