Fusion monsters tend to be among the most powerful monsters available to those who use them, and often outclass everything else either side will use. Though this card is useless against opponents who don't use Fusion (which may become more common now that Synchro Monsters have been released) against one who does, it is devastating. Thus this card makes an excellent addition to the Side Deck if not the Main Deck.
Note that since its effect specifically says nothing can be activated in response to the Fusion Summon, Dark Fusion users will be unaffected by Chthonian Polymer. They will not, however, be able to stop you from using it if they use Dark Calling.
This card is a great counter against "Power Bond", as you would snatch the Fusion Summoned monster with doubled ATK from your opponent, besides, they would suffer the drawback of sustaining heavy damage during the End Phase.