Personally, happy for the policy unless you are making a journey of 90 mins or less where flexibility is more likely required/happy to stand.
Journey planners can know (by being programmed correctly) which journeys this policy can be applied to. So a reservation can be made at the point of booking (however you buy your ticket) - and if one buys a ticket for a journey that there are no reservations available, the customer is advised that they may well have to stand or travel on a different service.
I can see it being sensible for long distance travel, but the trouble is that long distance trains also function as local trains for parts of their route. Essentially between Didcot and Bristol Parkway and Bristol
TM‡ there are no local trains. So we are all forced to use long distance services. Also I can't see it being popular for Reading Paddington people who have got used to fast services having to use local services if they do not have a booked seat.