The point of showing lowest possible spellslost to cast a spell is for such newcommer, somebody who dont remember those things to see his options on first sight.
IF you would remove all level indicators from icons, the only way to plan your next few moves would be to check every tooltip for every spell you have on bar ...
Like:
"now i want to Misty Step and then i want to use Shatter ... can i?"
> With no indication of spellslots, you would have to check both tooltips to know (since you dont remember)
>> But if both spells will have "II." on them, you see on first sight without need of searching anything that they both cost you II. level spellsot ... then all you need to do is move your eyes for few milimetters to see if you have two II. level spellslots avaiable ... and if not, again all you need to do is check all yours I. level spells, to see wich attack you will be able to use
That is the point of this suggestion.
