The probable reasons for sing the DNA Splicers are;
1. for the novelty/new Pokemon
2. because Kyurem-W and Kyurem- B have higher base stats than their original forms. Both Kyurem-W and Kyurem-B possess offensive stats and speed than their original forms, allowing them to be alot more scary :3
Additionally, you can revert them back anyway, so technically it doesn't detract from the points you mentioned since you can always return to having them seperate.