I would say that tritium, in the sights of a defensive carry gun, are unnecessary. Not useless, of course, but, unnecessary. My five reasons:
1. A personal hand-held light is fundamental, to me; more fundamental than the firearm. I also tend to carry a secondary/back-up light. (Neither of these are my iPhones, which, of course, could serve as lighting, for some tasks, though not defensive purposes.)
2. The best sight picture, in low light, is with one’s pistol’s sights silhouetted against an illuminated target.
3. Some of the time, my defensive firearm is equipped with a weapon-mounted light, a WML. (A WML is not a substitute for a hand-held light, of course.)
4. Target identification is vital, for most defensive situations. Tritium sights do not help, at all, in this regard. If I have a firearm with me, I probably have one or more of the lights mentioned in 1 and 3, above. I may leave the house without a handgun, from time to time, for whatever reason, but it is very, very unusual for me to leave the house without at least one sturdy hand-hand light.
5. Tom Givens has gathered a sizable body of data, regarding his numerous students’ defensive encounters. The data does not support the need for night sights, nor, for that matter, the need for WMLs. Criminals need to be able to see, in order to attack their victims, and if the criminal can see, so can the defender. (I do not believe that Tom Givens’ data shows that I should stop carrying a hand-held light, or that I should toss my WMLs into a garbage bin.)
Having said the above, I will add that, yes, tritium sights can be useful, in some foreseeable scenarios. The first, big one, that comes to mind, is that front and rear tritium can certainly help in aligning the weapon, in some lighting conditions. This could be especially important if one is using a weapon which may not be a “natural pointer,” in the user’s hands. Notably, at one stage in the life of my declining eyesight, I especially appreciated how the Trijicon dots assisted my scoring ability, when firing my police duty quals, a aligning the dots made it unnecessary for me to have sharp focus on the iron parts of the sights. (This was with a Gen4 Glock 17, in the lighting conditions on that particular qual range, during night-time quals.)
Some like tritium to only be visible, in the front sight. This is certainly valid, and I especially like this idea, with long guns being used at close range, in a point-shoulder-firing technique.
So, tritium can be useful, but I do not regard tritium sights as “necessary.”