The spirit of Minecraft?
Well, people have asked Notch in the past if he'd ever add shaders to it and he'd said that it would never happen as shaders were in his opinion, not in the spirit of the game's original intent, wether that will change in the future with Jeb now the main lead on it, I don't know. Maybe with the upcoming mod API. But in my opinion, they can be viewed the same as HD textures, in that they run counter to the game's vision if used excessively. Some run super high-res textures aiming to be realistic combined with advanced shaders adding bumpmapping of all things, and that I feel misses the point of the retroness of the game. GLSL shaders in particular. Not a knock on the shader at all, but rather a point of view on the whole idea of shaders in general in minecraft. I just find it ironic overall that people are using shaders on what's supposed to be a retro looking game. That said though, I find the dynamic shadows to be far more subtle than most shaders out there, so they're not so bad. They don't tend to stick out like a sore thumb compared to the rest.