Do you want your players splashing blue for countermagic or having to be more dedicated to blue? That's a pretty philosophical question. You probably run Spell Shrivel if you want decks that are primarily another color to be able to get a "hard" counter more easily, or if you like the idea of low CMC spells resolving more easily later in the game, which seems legit given how generally bad they're going to be. Tweaking the countermagic so that Doge always resolves later on could make for some interesting late games.
Taxing counterspells are also generally more fun to play with since they can feel more like they could have been played around, granted they're super powerful if played around, but I see a lot of "dammit, didn't play around it" from force spikes and mana leaks, instead of "welp, they had a counterspell". Granted at a tax of 4 mana, this is probably a super minor point.
Taxing counterspells are also generally more fun to play with since they can feel more like they could have been played around, granted they're super powerful if played around, but I see a lot of "dammit, didn't play around it" from force spikes and mana leaks, instead of "welp, they had a counterspell". Granted at a tax of 4 mana, this is probably a super minor point.