Really.

If, on the one hand, it's ecomonically inefficient for the defendants to have PDs, because they get worse deals for their clients when the charges are "minor", then the Rethugs' policy argument is:  "do away with the requirement for PDs in 'minor' cases, because the defendants get a better deal without them.  That "minor case" exception is the one which will swallow the whole.

Similarly, prosecutors (and cops) can tell defendants "you don't really need a lawyer.  Here's this scientific study and it tells you you'll get a better deal with us than you will through that skanky PD.  So, deal with us."

Moreover, if you look at the criminal justice system through the "administrative" rather than "trial" model - a distinction in which "administrative" minimizes the "jury trial" and "beyond a reasonable doubt" requirements and which Prof. Doug Berman over at Sentencing Law and Policy has explored over the years in the contexts of Blakely/Booker, sentencing-for-uncharged/unconvicted-crimes, and elsewhere - why have PD.  Economically, it reduces the system costs (dollars, cents and time) to not have PDs.  

And, let's not forget the distinction in ineffective assistance cases, where denial of the defendant's private counsel of choice results in a new trial, while denial of the defendant's public defender of choice is harmless error at best.  Scalia made a big show talking about wanting the best lawyer money can buy, during recent oral argument in a case on that distinction, but the distinction remains.

And, eliminating PDs would, of course, aid the administration of the Courts by acting as a "laxative on the docket" (h/t Grits for Breakfast, I think, for the phrase).  This surcease would not last long, though, as idle government employees would have their time sucked up by more cases, not fewer.

Of course, this all comes down to the ultimate (what some would call) amorality of economics - it recognizes only costs and profits, and decrees the former must be eliminated and the latter maximized, and now.  If a dollar sign cannot be placed before it, it doesn't count.  And, sadly, one cannot place a dollar sign before liberty and all those other, quaint concepts written in the Constitution and given lip service by the Rethugs.