Fear of Exposure

The additional cost runs convocation in reverse: rather than tapping creatures and lands to pay the mana, you tap two of them just to put the spell on the stack, and they stay tapped while the full still comes out of your pool. That is a steep surcharge for a 5/4, and it is the whole balancing act. A five-power trampler at this rate would be a text-free beater; green has printed that curve rarely and never so cheaply. Here the price is paid in bodies and mana access rather than in extra mana: cast it early and you have not just spent three lands' worth of mana but also frozen two more permanents, leaving fewer untapped attackers and fewer available mana sources on the turn you most want to keep developing. The card reads as an opening aggressor but functions as a follow-up: the tapping cost is cheapest once you already have a board, spare attackers or extra lands you can turn sideways without giving up anything you needed. Note that the cost demands creatures and/or lands specifically, so an artifact accelerator that is not also a creature or land cannot foot the bill. Trample is the payoff the additional cost buys back: an oversized attacker that refuses to be chumped into irrelevance, on the condition that you can afford to sit a beat and pay the toll.