What are the advantages of through beams over reflectors?

They cost more, so they must be better.

In my experience, thru beam sets are easier to keep from bleeding over to another finish eye and allow for wider finish areas.

A wider line is possible for sure but the main advantage is that you have a much smaller chance of an athlete crossing the line and not recording a finish. The yellow canadian suits from a few years ago were very reflective and athletes could cross a reflector line and the photocell would not blink. I have also seen duct tape do the same. Through beams will reduce this type of problem. Also the wide line is good for intervals where you may wish to shoot across the entire trail.

We actually had an opportunity to use a through beam setup to solve a problem we encountered ourselves. We were at Seven Springs PA the other day to race NASTAR. They have a typical ALGE RLS1n dual finish setup. The snow was blowing so hard it was tripping the finish every few seconds. NASTAR was being delayed by this. I skied down to the finish, took the second unused cell (since it was single course) and replaced the reflector. We were up and running within seconds. The through beam cuts through snow due to the fact that only one beam of light is present. Bouncing the light off a reflector increases the possibility of interference.