A good amount of the value does come from being double fixer. Obviously converge isn't a big deal for us (though a 10/10 Glinting Creeper on t4 and Kaleidoscorch are pretty exciting), but there's a fair deal of value in letting you cast basically anything while also doing a Worn Powerstone impression. Hell, it lets you hard cast Trostani's Summoner in UB reanimator with no other fixing (on t5 no less!). Even though the uptime doesn't let you curve out, I do think there are a decent number of decks that would be happy getting the fixing in exchange for mana.
What cards are you talking about here? Cultivate comes to mind, but that's green and forces you to run basics. Manaliths produce less mana and imo even the worst case of 2 mana every other turn is better than 1 mana every turn since you don't need fixing every turn.Quote from FunkyDragon »...And there are other cards that are better or more reliable for color-fixing.