Nobody can answer a question about intent except for the design team. I will say that I like the fact that the Plate doesn't have an easy answer to it like that. It makes it a much more useful tool for a mage who can't depend on having a multitude of Dispels and Nullifies.
The difference in wording though does have other implications as well. Even though it still amounts to you losing mana from your mana pool in order to have the effect you want, it is doing it in a very different way. Abilities that force you to pay for an effect, such as Armor Ward, typically occur during the Resolve Spell Step. Because the Harshforge Plate is increasing the base cost of the spell, this mana has to be paid during the Cast Spell Step. This means that the cost is paid even if it gets nullified, unlike the extra 4 mana for a Dissolve against someone with Armor Ward.
So, I don't know how much thought or intent went into the design of the spell, but as it currently stands it is actually very different from a "pay" mana effect, even though the result is similar, and it would be a bigger errata than I think most people realize. Not a huge game changing errata, I suppose, but it is more than simple word choice.