The only bad thing I see about the 1.5 is when the small cc records are being set. Ouch, thats going to hurt
And for most bikes, I really dont see them gaining very much more speed over their mile speeds. I don't think the Vmax will do much more in 1.5 then it will in 1.
The added time to run 1/2 mile even in a slow vehicle doesn't amount to much. It would probably mean little difference in the number of runs being made in a day.
How much speed is to be gained depends on the horsepower. At some point you run out of horsepower and another 10 miles isn't going to gain you any speed. For high horsepower vehicles on pavement OH HELL YES will they gain speed. 320 feet made a big difference to the blown fuel drag racers. Bonneville would be much faster if you could put the horsepower to the salt without spinning the tires.
At Bonneville you have to run 175 mph on the short course to be able to run the long course. If it turns out that the 1.5 mile speeds are no faster than the mile speeds for a lot of entrants, you could make a long course/short course distinction. It would take another pair of lights.
Higher speeds carry higher risks. At 1.3 miles on the slippery dry lake bed of El Mirage Pete Prentice ran 312 with a C motor. A 1.5 mile course could see 375+ speeds.