I would agree, though I’m sure their purpose was to display real time information along with the status of TfL/other GTR services and connecting services, so not sure why they only display adverts currently? Northern are just as bad with their recent digital train upgrades on Neville Hill trains, surely those screens can do more than show adverts.
Some of the Northern ones don't even show adverts, they just show a small digital clock in the bottom right hand corner. Furthermore, on 2 units I've been on lately, this clock has been completely wrong... in contrast, the ones on the Civity trains are quite good.
Back to GTR, I do agree that it seems like a wasted opportunity to fit these screens and then use them for the same role that could be fulfilled by a piece of paper - if that's what they're doing.
For what it's worth, I think the class 710 Aventras (London Overground) have screens fitted above the windows (where advertising posters would normally go on a Tube train, say) specifically to show adverts - so maybe LCD screens have actually become efficient enough that using them for this works out cheaper than people manually changing the posters?