Trap speed is an indicator of horsepower.
It's true that if you get a better launch, you're at higher speeds earlier. In road racing, the earlier you can get on the throttle the more speed you'll have going into the next corner. Road racing is just drag racing between corners.
The thingi s, if you look at say, a graph of the speed of the car through the whole 1/4 mile. It increases VERY quickly at first, and then it starts to level off. It levels off enough to the point that you can actually get a rough estimate of how much horsepower (rough estimate, but most ways of measuring horsepower ARE rough estimates aside from bolting an engine up to a dyno. Chassis dynos aren't all that accurate) a car has. Things like weight and frontal area/aerodynamics have an effect also, but that's why it's more of a rule of thumb thing.
For instance, I have a friend with a 1990 Camaro with a hopped up little 305 in it that has 243hp and 243 tq according to a chassis dyno. Those numbers may be inaccurate since he has a really loose torque converter (drag race thing) that skew with chassis dyno readings and makes them look worse, but they help ETs.
He regularly traps that car at about 102-104 mph at the 12.60-80 range. Last time we were at the track there were some awesome looking hopped up new Porsche 911 Carrera Turbos. They just couldn't get that car to launch off the line at all. They either spun the wheels or bogged it every time, and they were doing 12.70s. The difference from my friends camaro with the little 305 in it and these porches was that my friend was trapping at 102 mph, these porsches were trapping at nearly 130 mph.
So that's a good example of how mph can be a good rule of thumb indicator for horsepower. Because even if you get a TERRIBLE launch, the speed graph over a 1/4 mile run levels out near the top. So if you ruin your first 10 or 20 feet, you still have another 1300 to go, get the idea? By the time you get to the end, you're not accelerating very fast anymore, so the speeds will be relatively consistent most of the time even if your times are not. Thats because trap speed is determined more by how much hp your car has than how good you are at driving it.