To be honest I don't see much point in a Firestick as opposed to a full Roku as things stand. I personally get what you are saying, seeing as we pretty much have had a similar issue with the Apple tv boxes and the Nvidia Shield up until recently. (Apple tv has had a Now Tv app for years, but it has been a really poor app to use on a regular basis. Whilst the Shield couldn't be used to chromecast Now Tv until pretty recently.)
You can actually use ITV plus via Amazon channels, which is also on the Roku. (Still the same £3.99). It also has very recently updated the Amazon app to actually look a lot more like the other smart tvs out there, even though technically it was identical in features and use already anyway.
Think it’s also down to personal preference. I had preferred Roku for years purely for having all 3 Amazon, Now Tv and Netflix in one place, but I always disliked the Amazon interface on Roku. Typical when I start purchasing the Fire stick for other reasons including that the changes are made for Roku!
What ive noticed recently with the ITV hub through Amazon it takes forever for some programs to be added on catch up, whereas on the native ITV hub through a Smart TV it’s added almost instantly.
For me it’s purely down to device compatibly, Now TV and Sky should start allowing Amazon on their devices and vice versa.