In short, no. I don’t believe that every deck needs to run dual/tri colored lands. I feel like this is a misconception amongst most players; not every deck needs every available dual/tri-land to function. Rather than just state this as a fact, I believe it’s a good time to start showing some examples.
The cards shown in the cover photo of this article are all a part of my own 4-colored Kydele draw deck. In this scenario, it is necessary for me to run all ten fetches, six shocks, some tango and cycle lands, and a smaller number of basic lands. The reason for this is, as you can tell by the mana costs of the cards, is that it is very difficult to cast most of these. Plasm Capture especially, but in a deck that mostly hosts blue cards, it’s very difficult to even get Seedborn Muse out. My deck is very color dependent, meaning that more often than not, my ability to cast a card depends on the colors on the field I have to cast things. Additionally, I’d like to reiterate that my deck is a draw deck; I continuously try to fill my hand to cast things. As things enter my hand, I have to evaluate what mana needs to be set aside to cast more than one thing. Because of this, decks similar to mine need to be running a good number of dual lands to function.
On the opposite end of the spectrum are decks that are very color independent, meaning that there are fewer colored mana symbols on the cards itself. The unfortunate part of this topic is that the answer to ‘how many dual lands does this deck need’ is case by case. A deck that wants to cast Jarad, Golgari Lich Lord and Ashen Rider on the same turn is going to have a much harder time than a deck trying to cast Chalice of the Void for 12. The first deck is spending as much mana as the second one, but the second one uses eight less color symbols.
The concept of colored mana is something that all players know about and are aware of, but players need to translate this knowledge into their mana base. Using the example before, I would say that the first deck (Jarad and Ashen Rider) needs to be running a good number of dual lands, whereas the second deck (Chalice) could get away with just running basic lands. At the end of the day, it comes down to a judgement call. Do you want to cast things with lots of mana symbols or not?
If the answer is yes, then by all means load up your deck with dual lands. If not, then don’t. If your deck doesn’t pose the need for a solid mana base, why blow your budget there. If you’re unsure, then try a little experimentation. Leave the dual lands out, use basics instead, and try a few practice hands. If your deck struggles to put things on the field, then add dual lands until it doesn’t.
Another piece to this question is whether or not you have the lands available for use. When I say ‘available for use’, I don’t mean if you own them, I mean if you can place them in a deck. Do you have one copy of each shock and fetch, but want to run a Temur deck and a Naya deck? You can’t have lands in both places (unless you swap the lands around between uses), so find out where they are necessary. If the Temur deck feels starved of green mana, move the lands there. If the Naya deck runs low on green mana, put them in the deck. If both are always in need of green mana, that’s when I’d suggest buying the lands for each.
As mentioned before, this is a huge judgement call on your part. I personally don’t believe that all dual lands are required to run a functional deck, but some people do. This article is geared towards players that don’t own the lands and aren’t sure whether or not they should shell out the money for them. If your deck doesn’t need them, then don’t buy them. They’re a lovely addition to the deck, and if you find yourself in a position to get them, then get them, just know you don’t have to. It all depends on how the deck runs.
As you gain more experience as a player, these answers will become more obvious. I want to point you back to the suggestion I made earlier: playtest the deck. If something doesn’t work, then fix it. You won’t learn what works and what doesn’t unless you give it a shot. Nothing any article could say can give you more than just general knowledge; it’s the experience as a Magic player that will really teach you what to do.