For me it is, I run 3.9 also without reaction, including 3.9 workbench. The prefs programs are really just glorified editors and can fairly easily be replaced.
In most cases, there is no other equivalent to them that has the same functionality. Hand-editing the preferences files is not just a hassle, but also not a good idea as they are mostly undocumented.
Not just Preferences editors uses ReAction either but also HDToolbox, IconEdit, Exchange, etc.
Plus of course the fact that you would be unable to run lots of 3rd-party software.
All that just to save a few hundred kilobytes on your hard disk!? Seems like foolishness.