There are times when it's useful.
My mother in law was a nurse in the community in a mostly rural area, they would write (well Type) onto the notes the three words so other staff could find the right house on the right but if country lane without having to drive for a mile at 15mph as it's somewhere here and it's not easy to turn around if we sail past it (some lanes it's not easy to reverse up either).
Could it be better, certainly. However I would say that there are cases where it does work well. Like all things it's a tool and like all tools it's not foolproof.
Given that most emergency calls on the UK are from phones and people stay on the line, it shouldn't be too hard to recheck words of they flag up as clearly wrong.
Whilst it would be better if everyone knew how to read a map sometimes you've not got that clear an idea of exactly where you are and so a rough grid reference followed up with a W3W reference would give you a rough guide even if the W3W was wrong. However if it's right it would save a lot of searching, which if it's foggy or other low visibility situation can be very time consuming.
I think if people knew the phonetic alphabet, it might be easier to use What Three Words over the phone.
Delta, Oscar, Whiskey, November, Charlie, Alpha, Sierra, Tango dot Foxtrot, Romeo, Alpha, Golf, Mike, Echo, November, Tango, Sierra dot Charlie, Romeo, Oscar, Uniform, Tango, Oscar, November, Sierra
is the summit of a hill I am planning to walk up on Saturday, if the weather holds.
It should be possible to print off a card with the phonetic alphabet on it, if you can't remember by heart.
Where What 3 Words is useful is if you are able to communicate the reference by text or e-mail.