In a recent study, link found below, Yiannis Aloimonos found that orchestral players respond to the conductors movements, i.e. the conductors actually do lead the musicians. After reviewing the study, however, I found a few glaring problems with their methods. First and foremost, they used musicians from a certain region of Italy. The biggest problem with this is that there is a large possibility that cultural behavior in Italy could vary greatly from cultural behavior in other parts of the world. The Italian musicians may be more or less disciplined than other musicians of the world. I would be very curious to see what would happen if they did the same studied repeated in randomly selected countries.
The other problem with the study was that, although they used two different conductors, there is no real way to measure the amount of emotion and passion the conductors put into their job. I would guess that there would be a huge difference in musician reaction depending on the personality of the conductor. There is no way to know, however, the relationship with the conductors have with their musicians outside of the testing, and how well they are able to "feel" each other out while playing.
Study Link: http://www.npr.org/blogs/deceptivecadence/2012/11/27/165677915/do-orchestras-really-need-conductors