You see a Xenos Fighter ahead, blowing up transports. You're a Liberty Navy pilot, assigned to Colorado with your trusty Gunboat. You are allowed to cruise up to him to engage him.
The Xenos, feeling outgunned, decides to flee. He thrusts through the asteroids to Ouray.
You cannot cruise after him. You may attempt to catch him with thrusters, but you are too slow. Maybe next time, you can blow up his ship before he flees.
Situation number two: Your Liberator Light Fighter sights a Liberty Rogue fighter outside of West Point. You engage him.
The Liberty Rogue Fighter is clearly useless compared to your Liberty fighter. He thrusts away to the Badlands. You decide to thrust after him, keeping up with his fighter's speed (200 speed for fighters). You eventually catch him in the Badlands and kill the unlawful.
Edit: Megiddo, the cruise rule is to prevent shield running (more painful when capitals do it) and to allow fighters a way to escape capital ships.
Sure, they can cruise. You can thruster also, which is more beneficial in certain cases (cruise engines have to charge).