Today browsers are getting more and more features to really feel like normal apps.
One of the really exciting new features are Service Workers. They basically allow the caching of content locally which means that the website can even be opened without an internet connection (and loading the website in general would be faster and save bandwidth).
Along with some other features (like Push Notifications) the trakt.tv website could feel like a normal application.

I guess it would be good to go in this direction because it's easier to enhance the website instead of writing 2 completely new native Android & iOS apps. Also it would support basically all platforms that have a compatible browser.