Relatively few bugs are actually because a missing or stub export. Just because the export is there doesn't mean it behaves the way the application expects.

And not everything an application imports is actually called. That's why we have the @stub lines in the first place. It shuts up a lot of spam at program load time and still writes clear information when the function is actually called. FIXME lines/warnings aren't free - they tend to mislead users into thinking the crash is because of the last FIXME they saw.

Library imports also don't find COM interfaces used.