I think the blip has been removed for hostiles. There's just the sound now, maybe there's a blip if you're looking at it. It makes it semi-viable in theory, though I've seen nobody using it. Such as faking holding an angle with an AWP. Inferno mid, if you get smoked off you could fake a spam through the smoke with a decoy while relocating the actual AWP. It's incredibly niche and not that useful but a potential use for it.
The only other really practical thing I've seen with decoys is dropping your primary weapon, throwing a decoy and picking up the primary again, to make the enemy team think you're playing with only pistols. Any decent CS player would know what the enemy team is buying anyway, but it might be viable somehow.