1. We simply have no choice: considering the legal environment soon there will be no people willing to do this work anyway.
2. The lack of language skills and the lack cultural context knowledge severely diminishes our ability to extract valuable information.
3. Even if we are able to extract intelligence we are very seldom able to use it.
4. If "no prisoners except people considered legal combatants in the strictest possible interpretation of Geneva Conventions" is our officially stated policy, we may find a lot of volunteers ready to spill their guts in exchange for a more relaxed interpretation of their legal status.
Cheaper notwithstanding, there's two very good reasons to take fewer prisoners.
1. Fighting out of uniform puts civilians in danger. Under the Geneva Conventions, those who fight out of uniform in order to hide among civilians are a threat to those civilians, and may be executed on the spot. We choose not to do so, under our rules of engagement, but it is legal under the Geneva Conventions.
2. Those captured, except for the very senior leaders that are subject to harsh treatment, admit nothing and deny everything. Then, we're stuck with either throwing them back into the wild, or putting them in prison at our own expense.