[Suicide Junkie]

Using the energy they generate efficiently is always a good goal.
And the less they generate from fuel stores, the less they have to store in their hull.

However, entropy prevents you from recycling energy.

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You can easily have different detection methods for romulans or klingons.
The design decision of how to get rid of the heat generated on your ship would turn out differently for both. Internal politics, probably at the forefront.
"Design X was chosen because the company which designed it was located it the home province of the current governor", for example.