Blade does bring up a good point. However chances are Hammer may not know this. Depending on what happens perhaps Rothax may look to incapacitate any Namekians he fights in an attempt to get more info about the Dragon Balls. He could also try the whole "give me the DB's or he dies" bluff. Granted this would never work on Shitagi or the other Namekians but for the most part everyone else within the group doesn't know the rules behind them. That bluff on the right person could potentially get somebody who may not be as well versed in the lore to bite on it.
EDIT-And as another thing to note if Rothax manages to capture anyone he could just toss a temper tandrum in the event that none of the Z Fighters are willing to trade over DB's to save the life of that character. All he needs to do is a quick check of his own radar readings to make sure none are potentially in the way and then proceed to level a few cities as extra incentive.