It would look prettier, sure. I suspect that HTC chose the default smoke duration that they did for the following reasons:
1. It was long enough to see, but not so long that it will cause an undue FPS drop on slower machines.
and
2. They did not have the time to add a UI to change the smoke length (or decided there were more important things).
When writing stuff like this, there are time costs that are not always evident. For instance, they would need to add a UI to change the setting. Then they'd need to persist the setting by storing it in a reg key or settings file and knowing to query for it whenever smoke was turned on. Then they'd need to figure out if it was a value that should affect the length of smoke on other peoples FE (for instance, in head to head mode, should you be able to see the length of smoke your friend has set?) If so, they would need to alter the datagram structure for the net code to include that variable. Finally, they would need to test all of this.
Sometimes, a hard coded value is just the answer, especially when there are higher priority fix areas.
