I think that if Terramorphic Expanse had been printed much earlier in Magic's history I wouldn't be so impressed by it because in hindsight it would seem like such an obvious thing to help new players get access to inexpensive fixing in Constructed as a temporary alternative to costly duals. In Limited it is often a challenging decision to pick up a Terramorphic Expanse because it requires evaluation of your deck's current state and its needs.
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Constructed:
A new player is going to eventually learn that access to a variety of colors typically yields a stronger deck that takes advantages of all the strengths each color while mitigating weaknesses. Once they realize that they will see that the duals-du-jour of the of the format/rotation is ludicrously expensive.
Terramorphic Expanse is a cheap alternative for the duals you need. It is obviously worse but local supply and/or availability of money typically prevent any person from instantaneously getting the 8-dual-fixers that are in rotation and this card can fill the gaps nicely. It presents an opportunity to play a multicolor deck when otherwise a player would be shutout if they were stuck with only basics or short a couple copies of a dual. Terramorphic Expanse inflicts a tempo hit but sometimes the deck can afford to take it and in exchange acquires some much needed consistency to function and be viable.
Limited:
In recent Magic, rock-fixing includes cards like
Manalith or
Keyrunes, which felt like garbage most of the time.
Prophetic Prism is great and is a contender for great design but is beat out by the fact that it is almost never an option in Constructed. The common problem with these cards is that they take up a spell slot, which is an issue because unless they cycle, they don't exactly help you kill your opponent and you may waste the mana for the turn to get one out, possibly sacrificing board position. There were times in RAV where I found myself not picking or cutting Signets because there were better fixing alternatives and it was more important to pick up a threat.
Land-fixing doesn't waste a spell slot. Land-fixing isn't perfect either and cards like
Rupture Spire and
Shimmering Grotto are hardly playable because the former represents a brutal tempo hit and with the latter, substituing a basic of your splash color is almost always the better move because Grotto is a self-inflicted Force Spike on whatever card you're splashing.
Terramorphic Expanse is the best Limited color fixer because the worst thing it does is come into play tapped. It will not burn a spell slot. It represents any of your two primary colors as well as a copy of your 2-3 fixers for your splash. The tempo hit is still there but you can play it early unlike Rupture Spire and Grotto.
Cube:
Typically Cube has no room for this card. It's a strike against the card.
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It could be argued that
City of Brass is simply better, which it is as a card. But City of Brass and its update, Mana Confluence, are not always in rotation and their price tag and rarity put it out of reach for players. Terramorphic Expanse and its functional reprint
Evolving Wilds has been in Standard since Time Spiral, a sign to me that someone on the inside realizes the incredible utility Terramorphic Expanse provides to new players and occasionally experienced players who need to shore up their mana bases. Terramorphic Expanse helps the typical two-color Limited deck do what everyone wants it to do, "play my cards" and helps enable a third color splash, which supports designs such as
Crimson Muckwader and opens up more opportunities in Limited by simply improving the overall playability of cards.
When I was starting out I got a lot of mileage out of this card. There are a lot of factors that lead to victory and while having untapped lands is a large contributor to that I found that having a deck that worked was just as important and Terramorphic Expanse made those top-tier decks work on a budget.