Not a necessary (or very useful) thing in my opinion.
I run into this all the time. Start building a new monodeck by throwing in 20 cards or so, now sum all 20 cards together. Way too much quanta used, get rid of some cards. Deduct them from the original sum. Okay, looks better but running only 2 nightmares is unreliable and I can't really add any more. Better just replace them with something else. That leaves two empty spots, could use a pillar and a 4-5 cost card but just filling slots is silly. Better fiddle around some more. Okay, I can remove some of this and add some more of that, and uh... how many pillars was I supposed have to run again?
Add multiple elements, use more than 30 cards (try running 60-card monotime and keeping track of the costs.) and it gets out of hand even faster. As far as minor convenience tweaks go this would probably be #1 on my list as I experiment and attempt to optimize a lot. This would make the latter much more convenient.