We've pretty much all come to the conclusion here that for strength training, you should be training attributes, not sport specific movements. For example, instead of punching with weights in your hand, you should be trying to get stronger overall.
This is no different for conditioning.
Obviously, you need sport specific conditioning drills...padwork, sparring and other technique work will improve your conditioning to a certain extent.
The problem is with people's misunderstanding of the energy systems.
For 5 MINUTE ROUNDS, the aerobic system contributes the most ATP out of the three systems. Trying to use anaerobic glycolysis as the sole source of energy is a great way to gas out the first round.
Even doing activities at 100% intensity for 30 seconds, new research has show that the aerobic system is producing up to 50% of total ATP being renewed. Anything longer than this and the aerobic system will be responsible for an even higher percentage.
Your lactic and alactic energy systems don't change very much with training. They're controlled by genetics. However, your aerobic system is highly adaptable.
At the risk of plagerizing, I want to point out that these are not my ideas. A good place to find more information explained in a clear manner would be www.8weeksout.com
I'm not advertising, he just has a lot of good ideas and it's worth a look for anyone trying to improve their conditioning.