Most closed loop systems ...

Most closed loop systems (provided they are indeed closed loop, and not semi-open loop through losses), in spite of typically low corrosion rates in these systems, will have some debris and depending on the treatment option selected some corrosion product "throw".  The particulate matter cant settle into low flow areas or dead legs, resulting in accelerated corrosion under deposit.  Mainstream filtration represents a significant energy loss to the system over time, especially when differential pressures are not kept low.\

Side stream filtration with the correct media will improve system long-term performance, result in lower net energy losses in pumping, and as long as properly changed when back pressure is rising (if it is set up to be observable), or changed in a preventative maintenance schedule, then all is well.