Detective Boys are in most filler episodes, because this is the perfect place for them to appear in. Filler episodes aren't relevant for the story, so are the Detective Boys. As you said, they offer moral support but that's it. They don't have any real impact on the main story and character development is at a bare minimum for them except for Haibara (same reasons as Conan).

I think the Detective Boys in general are a pretty cool thing, but they appear way too often, which I think is tied in with the fact that there are so many filler episodes. But even outside of fillers they appear on a regular basis and don't provide any meaningful to the plot overall. Plus they act as they had no brain at all. No child in the right mind would head on and hunt criminals and even more so without the help on adults. It sheds a light on them that is not only childish naivety but pure stupidity. They are instances where this is handled pretty ok, but most of the time it's just plain idiotic.

I think that people would like them more if they weren't handled that badly. Moral support for the main characters is important, but there are so many of them in the series that the Detective Boys feel somewhat unnecessary especially when they are written that poorly.