If you proliferate multiple times, you don't have to choose the same set of players and/or permanents to get additional counters each time
If you danced with calamity a little too closely, and the total mana value of the exiled cards is greater than 13, the cards you exiled will remain there and you won't get to cast any of them
Most foil mythics are actually a lot cheaper than their scarcity would suggest, in large part because most Magic players arent terribly interested in premium cards
While it is better to pay the one mana most of the time, sometimes you just can't if you want to make any kind of progress on your board state
Theres an abundance of ways for you to untap Szeras and sacrifice a few creatures in the same turn to generate a ton of mana since its an artifact, but those creatures need to have cost a lot in the first place
Second, we didn't have the resources to playtest all these new mechanics like we would a normal set, so sticking toward known things makes it easier for play design (technically then development) to wrap their mind around it