Not when it is causing overcrowding. Once it can be 8 or 12-car, they should start selling them then.
I don't think it's quite as simple as that; another factor is that WMT have a huge issue with unused seats. Sometimes they are taken up with luggage due to either the unwillingness of passengers to use the (deep, well-designed) luggage racks on the 350s, instead surrounding themselves with their belongings meaning people can't sit down, or people carrying way more luggage than is technically permitted under the conditions of carriage. I recently saw a couple taking up not just the bay of 4 they were sitting in but also the one opposite with all their stuff whilst people were standing. (The guard came through and did a ticket check and didn't say anything!) People also seem very fond of lying down across 2 seats on the 350s for some reason!
These issues often create the impression the trains are fully loaded when they are not. It wouldn't be difficult at all to crack down on these problems.
Personally I think cheap advance tickets are good thing as they enable people to travel who might otherwise struggle to afford to, and they take a large number of cars off the road.
Anyway, as you say, it'll be better soon when they start running longer trains on the Trent Valley route. Perhaps they could also stop running 4 car services up and down the south WCML at weekends!