(B)  Availability of Prosecution Witnesses.  If an essential prosecution witness is subject to deportation, s/he may be deported prior to trial, or it may cost the prosecution resources it does not wish to expend to detain the material witness here in the United States so as to be available to testify against the defendant.  Other means of preserving testimony of prosecution witnesses, such as depositions or preliminary hearings, may not have the same convincing force as live testimony and may therefore weaken the prosecution’s case.  See § 6.50, supra.

CRIM DEF - COLLATERAL CONSEQUENCES OF CRIMINAL CASES BIBLIO
American Bar Ass'n Comm'n on Effective Criminal Sanctions & Public Defender Service for the District of Columbia, Internal Exile: Collateral Consequences of Conviction in Federal Laws and Regulations (January 2009), http://www.abane t.org/cecs/internalexile.pdf