I'm guessing you mean "integral PRDS" as a combined package of a "pressure reducing and desuperheating station" and they recommended this between very high pressure and high pressure, between high pressure and medium pressure and between medium pressure to low pressure steam headers.   

It is always necessary to provide a by-pass arrangement for the pressure reducing valve (PRV) for maintenance reasons.  Most design standards follow this practice.  The pressure reducing valve would require periodic maintenance and need to be isolated or taken out.  If there is no by-pass arrangement, then it makes very difficult to maintain the pressure reducing valve.  

If the combined PRDS package already includes a by-pass arrangement to isolate the PRV for maintenance reasons, only then additional by-pass arrangement may not be required.