I used the shower facilities in NRT upon arrival in the past and they're quite nice.
They're set up as individual cabins, similar to what can be found at any decent lounge shower rooms (except that they're clean and there is actually hot water, unlike at some lounges), nice Shiseido soaps, nicely lit hallways, etc...
There is a time limit (was it 1 hour?).

At HND you get a voucher for a free drink at the coffee place next to the showering facilities, don't know if it's also the case in NRT.

The arrival lounge has been a work in progress item since a very long time at JAL.
I think that a small facility at both NRT and HND with a handful of showers would be beneficial for the few who are not in a hurry to head into town.
There are people who want to sleep all the way into arrival, especially on short 9 hours hops like HEL-NRT. It's nice to be able to wake up and refresh oneself before heading to the hotel or meeting.
People who need to head directly to a meeting would also appreciate this service as it's easier to open your laptop and check papers in a lounge and do a final prep before a meeting.