It gives you more casts for the Reservoir gameplan and with the grim monolith + helm ramp, Mox Emerald, and Eternal Witness, it completely locks the game out with a recursive loop
Unless you go through the trouble of specifically placing more +1/+1 counters on her through different means, Arcee, Acrobatic Coupe will always only be a 3/3, while Arcee, Sharpshooter will likely only deal 1 damage each activation
It's also a key card in many cubes, so if you plan is to use Eternal Masters as a starting point to build your own cube, you probably should pick up at least a one copy now to hedge your bet against a future increase
While you do not have to replace all of these, these are the best options for cuts for some of the following cards
It's also great for commander recursion, though it would give me a stomachache to send my commander to the graveyard and then try to trigger this
But then Planar Chaos arrived with different versions of cards from alternate timelines and in one such timeline Akroma was an angry Red Angel