Viread, in combination with two other HIV drugs, is the standard first-line regimen for HIV and is used in more than half of all patients treated for HIV in the U.S. (Note that Viread is a component of the Truvada and Atripla combination pills.)
Clearly, a patient who takes Viread as monotherapy for an extended period in order to treat HBV increases the likelihood of developing resistance to Viread before it can be used in subsequent therapy for HIV. If this occurs, the patient ends up having to take a less-potent and/or less tolerable HIV cocktail that does not include Viread.