I don't know if I have any good arguments against limited prescribing powers for pharmacists, but just wanted let you know about my positive experience with this kind of thing:
My primary care continuity clinic is at the VA, and there is a Pharmacy clinic there specifically at the disposal of the primary care physicians that I refer a ton of my patients to. I think it's a wonderful idea. The typical way we use the clinic is when we have a patient that we're having a hard time with BP control or lipid control, so we send them to the Pharm clinic to work on ramping up their BP meds or statins. It really helps me out a lot because my clinic is always booked at least 4 months in advance, so the Pharmacists can work with my more high-maintenance patients on a more frequent basis while they're waiting to see me. As a caveat to this, however, is when I place the referral, I have to give very specific instructions to the pharmacists as to which meds the patients should be on, and which ones I want adjusting, and what the patient's goal BP or LDL should be. The pharmacists then have free range in checking labs, seeing the patients in their clinic, and adjusting the doses of the meds I asked them to alter. However, if a patient has maxed out on a med, or needs to change because of side effects, the pharmacists contacts me in order to approve any medication change. That way I'm not out of the loop, and can still direct the overall care of the patient. Oh yeah - and I still continue to see these patients regularly - I usually send them in the first place because I'm not physically able to see them as often as they need to be seen.
There are other things this clinic does, but the function I've listed above has been the most helpful for me, personally. I think the system really works well, as long as there are very good lines of communication between the pharmacist and the physician.
I guess that reminds me of a potential argument against - if the pharmacist changes patients' medications without informing or getting the approval of their physician, it can lead to very disjointed medical care, and potentially hazardous interactions. (ie, if the physician doesn't know a patient is on X medication, and then prescribes med Y, which is harmful when taken with med X)...
Just my 2 cents...