There are a couple of schools of thought.
As Ray said, the enclosed canopy is supposed to ensure that there is no intrusion into the cockpit, no matter the circumstance.
Some feel that a driver could be trapped if the car is upside down or against the wall and on fire, but with the open cockpit would still be able to squeeze out.
It does weigh30-35 lbs. more. If you have a light driver, not a big deal at all.
If you have a heavier driver, there is the penalty of not being able to move ballast for balance and stay at the minimum weight.
When it first appeared some thought it was an aerodynamic advantage. After some testing, a lip was added to the back, and the open cockpit cars raised the windshield to equalize the two.
At one point the top ten speeds all time were all held by Brittany (canopy) and lately the fastest car out there has been Salinas (open).
So, you decide.
Alan