1990 200 TQA..ISV WOT WGFV.. When and why

[Nicholas Lawrence]

    I suspect my ISV is sticking, I've tried cleaning numerous times to no
avail. Idle is erratic high and will go down if I unplug ISV.
    Question:  when does the ISV work and why not leave it unplugged? runs
OK in that mode.
    The WOT is open at full throttle (no continuity) and as I understand it
has to function in order for boost to be controlled by ECU. With analog
gauge I get only 4psi and bypassing the WGFV brings 12 plus and fuel cut off
so it is capable of boost.
    Question:  Is there a reason for not "making" the switch full time and
letting the ECU handle boost?
    And are these parts dealer only?
    TIA
    Nick   90' 200 TQA