In theory, there could be decks that don't want Glasspool Mimic (ones that have almost no creatures), but even with just a handful of creatures worth copying in your deck, it's significantly better than the Island it will likely replace
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1 Karumonix, the Rat King
To top it all off, it even had a combo finish with Altar of Dementia (utilised as a sacrifice outlet) and Bridge from Below (utilised to generate zombie creature tokens)
Frozone Super Slick serves the same purpose, and his Evasive keyword will be useful early on
This hybrid approach separates competitive deck-building from collector's joy, and it's the strategy most experienced players settle into after a few years in the hobby