The idea that something in a different media must be a faithful reproduction of the original is an idea that I hope dies a quick and painful death. There is nothing so smothering as amother'sfan's love.
If you read the book, and you liked the book, enjoy that for what it was. The movie is not the book. Any movie will not be the book. It is always the wrong question to ask, "Did the movie succeed as a successful representation of the book?"
The question should always be, "Did the movie succeed as a movie?"
Did I like the books Naked Lunch and Tristram Shandy? Yes and yes.
Did I like the movies Naked Lunch and A Cock and Bull Story? Yes and yes.
Were the two movies faithful adaptations of the books? Not even close.
You're not wrong in one aspect; cross media adaptation does mean things have to change to mesh better with the medium. Aragorn for example being more conflicted and doubtful towards his destiny than he is in the books works better in the condensed narrative of 12 hours of movie.
But, the issue of whether or not something is faithful to the books is actually more just an issue of authenticity than it is accuracy. Aragorn's personality change is still authentic to the feel of LOTR and its greater themes, even though it is substantially inaccurate to the original character.
I personally don't know enough about either version of Starship Troopers to make a assessment on whether the changes still align the two works or not, but I think the issue is ultimately still the same. If the movie isn't authentic to what the book was trying to do, then it isn't really a good adaptation, even if it might be valuable on its own.
After all, Forest Gump is nothing like the book, and that is indisputably the best thing the movie could have done.