Not at all. Even average defense will make a noticeable dent in the damage you do to most enemies with split damage weapons. Weapons that only deal one kind of damage are all-stars for this reason, if you have the stats to make them really work for you. The rule of thumb I've used in the past is that 100 split AR is roughly as good as 75 single-type AR, and at 40 dex the Uchigatana +15 is way way above that. Plus, normal weapons can be buffed, which is especially good with a quick weapon like that one.