Decals are more useful for small, undirected stains and spots. Things that nobody would notice aren't rotated properly, leaking is less likely or less important, and only add a bit of realism to a map. Drops of blood, dirt spots, pigeon poo (yes, there's a decal for that), etc.
They're less useful in TF2 IMO cause you don't see a whole lot of grunge/realism in a TF2 map. Plus most of the TF2 overlay textures aren't scaled for decal use anyway.
I tend to avoid decals lately in general, also due to how they leak onto other brushes.
Kep