That would very much depend on the situation and the people involved of course.

Someone with a deep and practical (practical being the important part here) knowledge of chemistry at the right place and right time? Could be extremely useful - he should have a leg up on the local alchemist.

A doctor, again with a good practical edge. Just being aware of bacteria and it's dangers, and using cleaner techniques would have changed everything. Discover antibiotics? Pure magic.

Joe average? Computer programmer? Probably fall into the "idiot" category unless you're good at killing people or politics.