I think the only reason why you would not try to debulk the original tumor is because you think it will not help anyway. However, Allen mentioned several studies which reported significant improvements after the primary tumor was removed.
In addition I want to mention the study by Andy C. M. Won et.al.
Primary treatment of the prostate improves local palliation in men who ultimately develop castrate-resistant prostate cancerThere are the following objectives for debulking the primary tumor:
Mr. Won reports that removing the prostate will avoid bladder outlet obstruction and ureteric obstruction which will happen when the primary tumor continues to grow.
Further, he also reports that patients who received local therapy to the prostate showed a longer mean time to develop castrate-resistant prostate cancer (8 years) compared to patients that did not (4 years). (Page E252)
George