After looking at the list of CDPs as it appears on Wikipedia, I believe that most in the larger metro areas of Detroit, Grand Rapids, and Lansing are significant enough to be included. More distinct rural/semi-rural areas should also receive a separate designation.
Doing a few quick searches, most if not all of the CDPs at least in the southern lower peninsula are already designated on the Waze map--even if that designation is not commonly referred to in the area. Many are major communities or suburbs, and others are not in common use, mostly because they may not be served by a post office with the same name. With that in mind, I would say ideally only include them if they have at least one official zip code where the post office uses the same name as the CDP.
CDPs around major metros like Detroit (including Oakland, Macomb counties) are readily identifiable as major or minor (albeit unincorporated) communities: Harrison Twp, Clinton Twp, Shelby Charter Twp (although it's just shortened locally to Shelby Twp), Redford, etc. Lansing has Okemos, Holt, etc. Grand Rapids has Allendale, Jenison, Comstock Park, Byron Center, etc. Even Jackson has *cough* "suburbs" such as Spring Arbor, Napoleon, and Michigan Center that no one locally would classify as Jackson. These CDPs all happen to have postal names that correspond with the CDP name as well.
Rural CDPs would be similar but for the opposite reason--they have a post office but cannot be served by any larger neighboring population. Places like Indian River and Mio are well known even if they are little (and even if just because they are mentioned on signs on I-75). They could easily incorporate but haven't.
The "grey area" seems to surround mid-size cities such as Kalamazoo. I actually grew up in Kalamazoo near the Eastwood CDP, and my parents still have the same house there. "Eastwood, MI" appears on the Waze map. In local use, Eastwood is just a neighborhood or part of the "east side" of Kalamazoo Twp. It is served by one "Kalamazoo" post office and is no different from other areas of Kzoo such as Milwood, Westwood, etc. Milwood is not a CDP, but Eastwood and Westwood are. Wikipedia even says "The area defined as the CDP may not correspond exactly with local understanding of the Eastwood neighborhood. The CDP may include nearby areas that locals would not necessarily identify as Eastwood." A similar sentence appears in the entry for Westwood. In cases like this, it may be really confusing to direct someone to Eastwood, MI or Westwood, MI because everyone says they live in Kalamazoo Twp or just Kalamazoo--mostly because that's what they see on their mail, or that's the entity to whom they send their local taxes. Another example from around Kalamazoo is "South Gull Lake", where everyone would simply refer to it as Gull Lake, which matches the name of the high school and all local services. Finally, there is a CDP for "Comstock Northwest", which is distinct from Comstock, the actual unincorporated community. Having never heard of the former designation before this morning, I would just get directions or send mail to "Comstock" instead since as with other CDPs in this grey area, Comstock Northwest does not have a separate post office. Of course, maybe this is just an historical problem with how Kzoo has handled their post offices, and the rest of the state isn't as screwed up...