What happened to the shekalim of those who lived outside of Jerusalem? How did they get all the coins to the Temple? Worse, what would happen if those coins were lost or stolen on their way to Jerusalem? Who owns the shekalim while en route? And if you're going to convert the coinage into that which is worth more, why use gold and not gems? All of which revolves around the level of responsibility of the people delivering the coinage, whether to the people who gave the shekalim or to the treasurers if the Temple... It depends on when in the process they are called to account. Also: Appointing someone to make your contribution of machatzit ha-shekel, and instead, he submits those funds for his own machatzit ha-shekel - how has he desecrated this Temple property? For that matter, in any case where the consecrated item (Ma'aser Sheni, for example) gets used for machatzit ha-shekel, you still shouldn't do it, but it's less severe than personal use. Of course, mishandling Temple funds is awful, but the fact that it could happen makes it necessary material in the Gemara here. The fact that the court could come after (most) people for collateral if he hasn't paid up his machatzit ha-shekel becomes salient.