You have 2 scenarios, and neither make it useless.
1) You use V-sync, and you get 16.7ms and 8.3ms frame times. While not terrible, it is not as smooth as 10ms would be.
or
2) You go without V-sync and accept tearing. While tearing isn't as noticeable at high hz, it is still noticeable.
So no, 100hz does not make G-sync/Freesync useless. It just makes it less needed.