tl;dr The question should be pretty self explanatory.
I have more SD/SE Tyranitar and B/C Tyranitar than I have dust to properly utilize. One of my old standbys - and first Tyranitar actually - has B/SE. At this point I feel like it would be better as B/C since I have less of those. Any reason keeping a mixed type set would be better than monotype?
No, the only real reason I can think of it really situational: Vs lugia in partly cloudy weather, but even then the smackdown sets would be better
I'd convert it to Bite/Crunch, it's the set that made Tyranitar, and seeing as the Regi's are out, Gen IV's imminent, and with that brings Giratina, and dark Tyranitar's likely going to be the only thing able to deal with it easily, not to mention the EX pass Mewtwo raids
Not much use for the mixed set in raids.
For gyms, one argument is that the mixed set does more damage than the dark set on flying targets, more dodge friendly, as well as hitting better neutral damage as well (almost negligible), at the expense of energy waste and charge move flexibility.
But there are better options for common flying targets anyway (dragonite and raikou) and sometimes even plowing it through with machamp is faster than switching into Bite/SE Ttar and waiting for the stone edge to hit.
