Some of the other issues related to this, listening to the comments and questions of cops discussing it ...
Ankle-carry allows for the Secondary to be accessed much more quickly in situations where the person is seated behind the wheel, especially if they hadn't taken the time to adjust their seat belt, and car coat, properly when they got in the car ... Or, the seat belt and car coat became twisted and displaced during a violent series of movements while driving code, and coming to a sudden stop ... and then the suspect is suddenly outside the vehicle, armed & SHOOTING ... while you're belted in your car. This isn't a hypothetical situation, as I know someone who found themselves in this situation, and unable to "extricate" their holstered service weapon while getting shot at.
Some cops apparently only consider their Secondary to be primarily intended to replace the Primary weapon when it either runs totally dry, or else is disabled or unavailable for whatever reason ... and plan on being able to draw & present their Secondary with their dominant hand. They seem to think of it as more of a "second primary" weapon. Placement which emphasizes dominant-hand use seems to be their choice, and weapon type selection seems consistent with their primary weapon.
Other cops seem to think their Secondary weapon is going to be mostly used when they're unable to access their Primary weapon, or else they're fighting for control of their Primary ... and they plan to be able to use their non-dominant hand to draw and use the Secondary. In some instances, you'll hear discussion of a small revolver being considered preferable for such duty, since a small platform semiauto, used during stress in a non-dominant hand, MIGHT involve a bit more potential for the weapon to experience a "grip related" malfunction if the operator doesn't get the "proper" grip on it during those critical moments of high stress, and perhaps violent physical exertions.
Some folks seem to think that just "having a Secondary" is sufficient, and apparently don't think much beyond that ... not about the caliber, weapon type, method of carry, etc., etc.
Some folks seem to think that Secondary weapons of just about "full size" dimensions are preferable. I've seen Commanders & M66's carried as Secondary weapons ... and other folks seem to think diminutive guns and calibers are sufficient.
Some agencies have guidelines for the method of carry, placement on the body, type & caliber of Secondary weapon ... and even issue them.
Listening to different folks explain their thoughts and desires regarding Secondary weapons seems to reveal that the concept and envisioned purpose of Secondary weapons can vary somewhat among folks. These varying ideas can change not only the opinion of what weapon/caliber is better suited for different needs, but the method of carry considered, as well.