I would say decoy is good in a siren, songs, and a fm, forcefield and spells like charm, etc.
I ran one in my siren for a long time and it woukd immediately get seeking dispelled. Not sure how many people run 2 so that worked out well!
But since tele bombs release, as a fm i would much rather stick one on instead of decoy. If its nullified or seeking dispelled, you spen 1 sbp and they most likely spent 2. If it sticks then you have a useful spell instead of a decoy laying around.