The IMEI is trackable and triangulatable geographically as it moves around the network. So it still helps determined actors to find the phone, and therefore maybe the user.
What IMEI gives you is the ability to track the same phone across SIMs.
The main scenarios where IMEI should be interesting to law enforcement are: stolen phone situations, criminals that keep getting getting new burner sims while continuing to use the same phone, or people that keep prank calling emergency services without having a SIM installed.
In nearly all other cases, the ISMI is far more useful identifier. And obviously the phones in question all have unique ISMIs or the network would be having, err.... great difficulty with all of those phones having the same phone number, and being able to route calls.