Doing Bi and Tri work together should be good since they are antagonistic muscle groups (push vs. pull)
A theory about doing Back and Bi together is that while working your back your Bis are being hit hard. I have done training blocks with back and bi together but always make sure to hit back first since it's the larger muscle group and finish up with biceps.
Another theory about working triceps on a separate day from chest or shoulders is that your triceps really don't need that much work. Chest and shoulder days already work triceps pretty hard so some believe adding a third day to tris is overkill and doesn't allow for rest and growth. I've worked all of the scenarios into different training blocks and have been successful with all.
I don't believe one should ever stick with one combination of training. Split training up into blocks (ie Hypertrophy, Mass-building, Strength, cutting) and try some of these theories/myths for yourself.
In Health,
DM