you might not care about bandwith but most people care about load times when accessing nzbdrone remotly rather than from the local network. I would say almost all of our user are running nzbdrone using a home internet package which have much slower upstream than down. for example I have 25 mbps downlink and 2.5 mbps up-link. in an app like nzbdrone where most of the traffic is images cutting down image sizes by half means your reduce your load time from 10 seconds to 5.
webp support in nzbdrone will happen regardless of trakt support. but we might have to offload the image conversion to our servers for the time being until trakt starts serving images in webp.