There are definitely pro's and cons. The obvious is that it will take away from clinic time. The way I understood it was that it would be incorporated twice a week instead of 3 times a week like it is in 1st and 2nd year. (I am not sure about how many hours. Currently it is two hour blocks in D1/D2 year)
What this means is that twice a week, one of your clinic shifts would be substituted with a PBL session. The clinic is split so that you have a morning shift (9-12) and an afternoon shift (1-4). Multiply this by 5 days and you are looking at 10 shifts/procedures a week. Now in theory you would be doing ten procedures a week but in reality no ones schedule is that full and at least a few shifts a week you are sitting around essentially doing nothing..maybe assisting someone or looking through old charts. So in reality losing two shifts is not a huge deal in my opinion because you can schedule the rest of your patients around those slots and will still probably have a shift or two that is empty. Also keep in mind the demographic of populations coming to a dental school for treatment, they are most often lower income so probably 60% of what you are doing are exams and cleanings with extractions being the next more common dental procedure because it is the cheapest way to get out of pain. Those that are able to will do fillings or crowns and rarely you will get a root canal because of the cost associated with them.
So as far as losing clinic time, i wouldnt worry about it. If you have cool procedures like fillings, extractions, root canals, etc you can schedule them on another day. If you have a cleaning...you'll be glad you can skip it because you will be so proficient at them in such a short amount of time that it starts to get mundane after a while.
The pro's I see of this is that learning about pharm and oral path is a great skill to have to be a well rounded doctor. I'll take a doctor that knows how to spot abnormal findings and know more drug interactions than one who has done a handful more cleanings or fillings.
The cons are well...no one really wants to go to PBL...I mean its not the end of the world but we all look forward to third year because we finally get out of the classroom and to have to go back will be such a drag.
Fourth year will be unaffected. Written boards are over after third year so there would be no point in having PBL afterwords and to have PBL in other locations wouldnt make sense for the school.