Outside of Commander, doesn't play well at all- Mana needs to be consistent- those are anything but.
Actually they're exactly like a dual as a 1-of. Which is lot of $$$ from tricolor decks. consider this mana base:
1 Karakas
2 Arid Mesa
2 Plains
2 Volcanic Island
2 Wasteland
3 Island
3 Tundra
4 Flooded Strand
4 Polluted Delta
This is Miracle. Now imagine:
1 Karakas
2 Arid Mesa
2 Plains
1 Legendary UR
1 Volc
2 Waste
3 Island
1 Legendary UW
2 Tundra
4 Strand
4 Delta
It's functionally exactly the same thing, but you shaved like 400$ from the deck which is huge. And if you wanted, you could add 1 more fetch and shave another dual, which while not perfectly optimal, it's still pratically the same as this mana base, and league better than using Shocks which are just bad (in legacy context of course).
Legenduals would make legacy mana bases much more affordable. Yes it's lazy, but since WotC seems to be unable to make good situationally better than duals lands, i see this as the most realistic option.
Situtationally better than duals lands would be the most interesting for sure. Imagine:
Koilos Strand
Land
When Koilos Strand ETB, your opponent lose 1 life.
T: add W or B, lose 1 life.
Karplusan Wastes
Land
ETB tapped.
T: R or G.
RG, sac, T: destroy target non-basic land
Cemetery of the Forgottens
Land
As it ETB, exile your graveyard.
T: add U or B to your mana pool.
T: exile target creature in a graveyard.
Bloodstained Peak
Land
T: add R or B to your mana pool, deal 1 damage to you.
T: target creature you control gain haste and +1+0. Sacrifice that creature at EoT.
Unluckyman Cavern
Land
ETB. When it ETB, look the top 3 cards of your library. You may reveal a basic forest or swamp from them and add it to your hand. Put the other cards on the bottom.
T: add G or B to your mana pool.
All probably too good for standard, but easily printable in precons and would open interesting deckbuilding possibilities.