There's always the possible assumption of PMR or neuro for the less competitive candidates I suppose.
But yes, I don't know of many people who apply IM-prelim as a backup, and it would look strange. The different places strategy would be one way to go about it.
The other thing to take into account is that prelim programs are frequently (usually?) MORE competitive than their parent IM program. In some places, a LOT more competitive. Your competitors for categorical IM are your typical med school graduates with a wide distribution, while competitors for IM prelims are almost all people who are applying ROAD, Rad Onc with the PM&R and Neuro people thrown in.
There are plenty of community programs known as being on the "cush" side where the credentials of the prelims/transitionals are far and away superior to their categorical seniors. I can think of one I rotated at during medical school where the TYs were all AOA people going on to do ophtho at harvard (or equivalent) and the categoricals were 100% foreign grads, not even caribbean.
All-in-all, it's a very different reality than the surgery programs I'm aware of. The majority of people who do prelims actively try to avoid surgery programs, and there's a culture with a lot of people who couldn't otherwise match surgery doing prelims to try to get a spot through a back door. I can't say it never happens in medicine, but it is much more unusual.