Cautionary tale: The movie "Awakenings" was based on true events which took place in 1969. A doctor discovered that a new (at the time) drug, L-Dopa, would wake patients who had been catatonic for years. They were fully conscious and could carry on conversations, etc. The drug was horrendously expensive, but because it worked miracles, the hospital found ways to fund the doses.
Then they started noticing that as time went by it took more and more of the drug to be effective...
Eventually, no safe dosage of the drug would work any longer, and the patients tragically faded back into catatonic states, for the rest of their lives.
It would have been great to give this drug to Terri Schindler Schiavo.
That a drug appears to impart awareness to a person who would otherwise appear to be PVS does not mean that it should necessarily be used to continuously impart conciousness. At minimum, however, I would think short-term use of the drug could be very useful in diagnosing a person's condition.
It may well be that a person's condition is such that there is no known therapy that would provide long-term improvement. On the other hand, if a drug provides short-term improvement, that would suggest that the prognosis for long-term improvement should be excellent (since it implies that the necessary parts of the brain are in "operable" condition and will work if properly stimulated, even if the proper means of stimulation are not yet known).