I sort of thought this had already happened, so my apologies for not raising this earlier…
We should remove PatientSetService, and its Impl and DAOs, from openmrs-core in Platform 2.0.
The history of this service is that it was the original implementation of reporting/analysis functionality, in the pre-2010 era. This was before we realized that it wasn’t practical to implement something like this in openmrs-core (which would iterate slowly), but it belonged in a module (which could iterate quickly). So my understanding is that today these methods are buggier versions of code that was eventually added to the reportingcompability module, and used in the reporting module.
Anyway, I propose getting rid of this, because nobody should be using/trusting it at this point, and the best implementation of that same functionality is in the reporting module.
(I have created https://issues.openmrs.org/browse/TRUNK-4874 as a placeholder for this, though I haven’t marked it ready for work yet.)