I suspect the DMPs probably have to pay some sort of annual fee to remain registered as DMPs but I wouldn't have thought it would be per medical carried out, so I doubt it is to CIC's financial benefit to request more than one medical from a person. Maybe I'm wrong.
In family class cases, the medicals go elsewhere for initial review separately while the application you sent awaits sponsorship approval in Mississauga. My app arrived in Mississauga on Feb 27th and I did my medical on Feb 10th - I have a letter dated March 9th that originated in Nairobi and was sent to me from the London VO requesting further medical testing. I still have at least 3 weeks before I can expect sponsorship approval. The medical gets matched up to the app later when the app is transferred to the local VO, your medical doesn't hang around in Mississauga gathering dust with the rest of your app. So meantime for me, while I await sponsorship approval from Mississauga, I am able to sort out my additional medical requirements. This works out enormously to my benefit because it should mean that my app doesn't get delayed for months later in London.
So for someone whose VO takes more than a year to process, the applicant will at least have been able to progress the medical side of things during that time if any further testing needs to take place. I think that's why they have your medicals done upfront. It works out quicker in most cases since the medical stuff happens simultaneously while everything else is going on (or not going on, as the case may be

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I think in the case of skilled workers, they request the medicals at the end of the process only but for those immigrants, timing is supposed to be less of a factor. In theory, spouses are supposed to be reunited quicksharp, so I guess that'll be why they insist on the medicals earlier.