I'll put a hypothetical example forward, For this example, let's say you currently trap at 106 mph, and your 4th gear tops out at 120 mph. You decide to change your gear ratio so that your 4th gear tops out at 108 mph (a little wiggle room, since the gears will hopefully let you trap at higher speed along with better ET). Now, you've dropped your ET and raised your trap speed to 108 mph. Congratulations! However, if you add some power (pulley,intake, whatever you like) which hypothetically allows you to trap at 112 mph, now you're going to waste precious time making a shft from 4th to 5th, minimizing your gains. Now, you have to re-gear to top out at higher than 112 mph to get any use out of that extra power.

In addition, IMO, our cars are already traction challenged right now. If you keep an open diff and street tires, a gear change could simply mean an extra half-second smoking the tires instead of grabbing traction sooner with less gearing. Changing gear ratios means a LOT more fun driving the car on the street (because it FEELS faster), but it doesn't necessarily translate to actually GOING faster.