On the defensive, it can serve to save your creatures from opponents combat tricks, but thats far more narrow a use case
Rat Colony only costs two mana, making it easy to get onto the battlefield early and if you draw multiple, you can raise the power of each one
Since youre probably waiting a turn cycle anyway, there are better options
With this commander, there might even be situations where you can actually make use of its lackluster ability, perhaps giving a strong or unblockable creature the frog creature type so it can mill extra cards while slipping past or over blockers
Brew #3: Tamiyo, Inquisitive Student Now initially I skipped over Tamiyo because I initially assumed the plan would be to flip Tamiyo asap with a Brainstorm or whatever, but planeswalkers kind of suck in Commander: its impossible to protect her against 3 opponents, and none of her abilities are particularly good in multiplayer
Use the greatest power among Mutants you control as Vault 87's last ability resolves to determine how many cards to draw