That's a good question.

We know that pain stresses the nervous system, and that it also increases incidence of depression. Both pain and depression lower survival chances, as they both increase the chances for negative effects on the host such as sudden weight fluctuation, abnormal sleep cycles, and uncoordinated muscle control.

I have not worked in such a lab setting, so i'm unsure if any of the mice that survive experimentation are exterminated or repurposed.

If repurposed, one can imagine that a pain-reduction schema would be useful in such environments for the sake of species proliferation. Mice/rats better suited to the pain thresholds required of them would eat better, sleep better, and ultimately breed more.

Again, just an armchair theory and little more.