I don't typically carry non-lethal or less-than-lethal, but my rationale for that should address the OP's question close enough to on point.
First, LEO's usually have back-up present or on the way when they suspect a problem encounter. Even for a routine traffic stop, it is typical for a second unit to at least drive by and check status of the officer. I typically do not have back-up.
Second, a Taser typically only has one ranged shot, after which it becomes a contact stun gun.
Third, sprays either require aim, if concentrated, or are subject to wind, if dispersed. Also, courtesy of Uncle Sam, I have been subjected to the old "remove gas mask and recite phonetic alphabet" drill in a CS chamber. It was unpleasant; burning, watering eyes; draining sinuses; snot running from nose and dripping down chest; mild choking sensation. However, could I have continued a determined attack? I suspect I could have, though my ability would have been degraded.
I know the OP does not wish to discuss unarmed training, but that is also part of my rationale. I have a lot of training in evasion, escapes, takedowns, and controls. A person I cannot physically handle is most likely somebody who will shrug off non-lethal and less-than-lethal devices.
As others have indicated, the two best tools are awareness and avoidance.
But I still strongly believe anybody who carries should have at least some training in close contact / hand to hand, too.
Otherwise, good luck deploying any system, unless you see the attack coming early.