I’ve thought about this a lot lately and am changing my current duty line up to account for it. Sorry for the incoming wall of text.
My agency specifically prohibits the carrying of fixed blades for whatever reason, however they do have a pretty liberal BUG policy.
In the lethal force encounters that I’ve studied things rarely go according to plan... this seems to apply especially with defensive knife uses. I remember a P&S modcast where one of the guests had used a knife in a defensive situation and the number of strikes before his adversary finally gave up was something incredibly high. Ultimately he ended up doing time before later getting acquitted on appeal. I want to say it was One if the guys on the episode on the mindset of violence, but I could be mistaken.
I also think of the 2008 shooting involving Timothy Grammins that was highly publicized when it went down for the extreme round and hit count before the subject went down. I believe the bad guy was hit once in each lung, once in the heart, once in the kidney, in addition to taking 10 or so other .45 ACP rounds before he finally went down with three to the head/face.
With those things in mind, if I suffer a gun grab and end up with a guys hand pinned to my holster in an effort to retain my gun, or end up tied up on the ground with my strong side out of the fight and find the threshold for use of lethal force met, I would rather employ a j-frame or 42/43 with Chuck Pressburg’s philosophy of use long before I would a knife.
Tl;dr:
If it gets to the point where a back up lethal force option is required, shit has gone terribly, terribly wrong. I’d much rather have an off-hand heater than an off hand knife, but YMMV