The design team (Aaron Forsythe, Richard Garfield, Mike Elliott, and Tyler Bielman, led by myself) approached Dimir as the "library guild," much as we approached the Golgari as the "graveyard guild." As I explained above, this comes about because blue and black have more interaction with the library than any other two-color combination
Yes, spending five treasures is a lot, but Mind Control and similar cards are considered premium removal that usually costs a whole card
I also really like Walking Atlas and Scaretiller as backups to Patron for putting lands back onto the battlefield after being bounced with Moonfolk or other effects

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Overall, I hope the above overview can provide some hints of which starter deck may be more suitable to buy if you are looking to get into the game