Some people, deaf or hearing, are functionally illiterate. That means, they know the alphabet, and they can recognize and write simple words but they can't put those words together in a meaningful way when they try to read or write.
So yes, having "poor" English skills can be just as bad as having "none" when it comes to filling out forms, signing contracts, following directions, understanding a medical diagnosis, etc.
It can be very dangerous to substitute note-writing for interpreting, in certain situations.
