I don't understand where that is coming from. Please point out where the anime butchered the source. Point out specific examples so that I may know. The only noticeable difference is the fight with Caster happened on the bridge instead of in Shirou's house. And I think mind that at all. Shirou is portrayed as somewhat useless in this first half, more so than usual, but it's like that even in the novel. He doesn't start going protagonist until the certain scene between Archer and Rin. There are other smaller changes that are acceptable.
Because Shirou's character was messed up. And it's not just the Taiganapping scene. They cut out or altered most of his snarky moments with Rin which is really a heavy point to make Shirou over-all a likeable guy and if you can't make a character likeable then his complexity does not matter as no one is going to care about him. He's just going to be that one ''annoying typical shounen MC''. He's way too passive in the anime when talking to Rin.
They didn't include his inner thoughts in battles such as Rider vs Shirou and Archer vs Caster and thus have a completely off picture of his character. For an example, anime Shirou seemed like an arrogant idiot by telling Rider she's nothing compared to the other Servant and even he can take her on and then got wrecked by her literally a second later and had to be saved by Rin. VN Shirou immediately has thoughts about how he's finding it unbelieveable that he can keep up with her and that he should have been dead already multiple times, showing that he isn't arrogant and knows in what kind of a situation he is in.
Once again, he comes off as an arrogant idiot in Caster vs Archer because he yells at Archer to let go of him because he can take care of himself while in the VN he has thoughts about how he doesn't want to burden Archer with his own lack of competence. That is relevant to his hero complex and his lack of care for his own self.
It's not about his uselessness, it's about his likeability and character itself. Shirou isn't an arrogant idiot in the novel. His off portrayal in the anime is not a good thing because the whole story-telling of Fate/Stay Night and the topic it discusses heavily depends on Shirou and his character. If the anime can't replicate the Shirou from the VN, it becomes a bad adaption because Fate/Stay Night was never a story about the Grail War. It is a story about Shirou's world-view and ideals. It discusses the topic of heroism in increasingly cynical ways through Shirou's character.
That's why most anime fans think of Shirou as a stupid typical shounen MC while the VN fans love him. There's also the case of lack of info dumps and other minor things.
