The problem with this approach is twofold: a) At the margin, a few people doing this does _nothing_ to mess with big companies' data collection & analysis. But

The problem with this approach is twofold: a) At the margin, a few people doing this does _nothing_ to mess with big companies' data collection & analysis. But opting out also has the same problem, obviously, so at least it's not doing worse. b) In the absence of sandbox / selective sharing features like other commenters have mentioned, or you going so far as to _only_ keep fake contacts in your phone, using this approach requires you to also share your actual contacts with the app, thus giving away PII of unconsenting third parties. Yes, I'd rather blame the app developers for collecting this data in the first place, but I'd still prefer not to give my contacts away whenever I can reasonably withhold them.