But very useful on the right hands. Give Zoro FS and well...
However it's not clear how FS works yet. For instance, what if a character wields a level of basic CoO superior to someone else's who is also an FS user. Is their FS more useful then? I bet it might be the case yes. If that was the case, Zoro would get empowered by posaessing FS, differently he already showed how his CoO could overcome Luffy's FS so if he had that FS his CoO would not change.
For sanity's sake I think Oda would probably boost his overall CoO, therefore basic CoO augmenting FS.