It's not supposed to increase the number of polymerizations in your deck, that's just absurd. The purpose of this card is to search out poly, having said that though you really shouldn't be using this if you only nave 1 copy of poly also as others have pointed out this card is easily a dead draw if you're unfortunate enough to draw out all your polys with this in your hand
And that is exactly the card's purpose. The closest true replacement to polymerization I believe is fusion gate, although that has the drawback of banishing the materials, and allowing your opponent to fuse. Fusion recycling plant allows you to get polymerization from the deck or graveyard, but you need to discard first.
Just run an extra copy of Polymerization! Not really useful for thinning either. What if you draw Polymerization first, then this cards is a dead card in your deck.
(2) and with such condition, having only 3 polymerization is not enough to have consistency in those deck. To increase it, therefore other fusion supporting card is needed, like fusion sage, or fusion gate, hence making the spell card count to 5