No, scientists should not be the only people to make policy decisions regarding scientific concerns, because there would end up being very political decisions. Decisions would be made based on who has the most power. It would also come down to how much profit can be made from each particular activity.
I do not believe scientists should be the only people to make policy decisions regarding scientific concerns. I believe people are capable of making these decisions when the advisers to inform them about the issues. For this reason it is acceptable for scientists to simply be advisers to leaders, they shouldn't inherently be expected to participate in a political sense.
It's never a good idea to have policy decisions made solely by the same group that will benefit from them. Any highly educated and trained group tends to have a laser focus on their particular discipline. When it comes to making policy decisions, an assortment of people from different walks of life need to be included in the process to widen the perspective.
Scientists should not be the only people that are able to make policy decisions regarding issues that deal with scientific concerns. This is especially true when in general that these issues will deal with the general public at large. If it is going to deal with the entire population shouldn't certain people have a say too.