First of all, a bit of background, we need to take into account the three ways (so far) one may use magic in this world, such as such as having a lantern (an object which emits light, as opposed to a fire torch or a candle).
- The magician produces the light / lights the object by himself.
Light may come from his own hands, be a flying ball, or radiate from his staff (think that image of Gandalf in Moria). Even though the wizard may have made the light appear from the lantern he is holding, the lantern itself is not a magic item. The magic making that light appear is coming directly from the magician. The spell is simple, useful and requires little magic. How much you exactly spend will depend on how far you want to see, the lumens you are making our of the lantern, and for how long. A wizard will unconsciously be automatically tuning it as needed. And when he no longer needs the light source, he simply stops the casted light. Since a break of ½-2 hours is enough to recover from powerful spells, something like this would probably be restored in less than five minutes.
- There is a permanent item embedded of magic.
In this case the lantern would be a magic artifact. As it takes a lot of effort to create, a lantern would probably not be a good use of resources [it makes for a nice 'school' assignment, though. Apprentices can startpracticing with artifactcraftship, and light are good to show in a visual way their magic (if you don't think that would be needed, then you never had to explain to an apprentice that they are not casting uniformly around themselves, OTOH "see your lantern is barely lit from this side" is understandable to their previous, materialistic experience)]. Maybe, for a light tower, it would make sense to create an artifact between several mages, rather than having multiple wizards taking turns, as they need to do during stormy nights. The creation ritual is much longer, but less demanding than what would be needed for producing flashes during a bad night.
- Using a runic-band-based item
These are the items you ask on this question, and are completely different than the previous artifacts. In fact, the name is also completely different, but non-practitioners "don't get it". This is an abacus, the other an electronic calculator, yet for them both are "that counting thing". Exasperating.
When would that be useful? Let's suppose this night the wizard is studying a grimoire and preparing some spells. His wife wants to read poems in the living room and the servant needs to go to the larder to pick/leave some items. Some candles would do that perfectly, but that would cause a lot of drama, as if living in a house with a wizard meant that people became allergic to non-magic light. Yet, he doesn't want to leave his grimoire to go lighting the path to the cook. Or be lighting the poems. Even if he didn't need to be in the same room as the targets, he would still want all his power available, not lighting other mundane things.
An item powered by a muggle nicely solves all of this. Most importantly, it doesn't keep his own magic captive. It also opens the door to a commerce of magic items which wouldn't be possible before (a king or a lord might have an artifact, holding magic from his own wizard, but this allows everyone to have magic items, as they would only consume their own magic).
Enough background about the different types of magic actions. Why do runic band items have a concurrency limit?
It is established that people have a container where their magic is stored. Powerful mages can store more of them, and it is possible to enlarge it somewhat through practice. When cosmic rays go through people, their magic energy may get stored there (if there's space for it). There is no much difference on the ratio of different people recharging (although it is non-linear, the more full your tank is, the harder it is to fill it up).
However, when it comes to emptying it, there is a single "faucet" through which the magic comes out.
On the first case, the magician is actually the one opening the faucet as much as he wants (up to the maximum), and pushing the magic -with more or less force- to go out.
On the second case, "a portion of the magic container" (to express it in lay terms) has been moved into the object. That is also restricted to the amount that may flow in the "open" position (it's like taking out magic with the wrapper still around it), but enchanting a permanent object is a really slow ritual (partly due to this). Once placed in the object, it is possible to use it with little to no limitations.
However, a runic attachment is like connecting a pipe to the faucet. The pipe connector is rigid, has a fixed diameter and forces the faucet to be open (note the magic wouldn't go out on its own, as it's too dense).  Wearing three of these items look like this:
(Image by Koko90 and Antonsusi)
It doesn't matter the amount of magic that the user has stored inside them, there is no space to connect another runic-band-item. A wizard or a witch can have multiple spells active simultaneously, dynamically providing more or less magic to each of them, but these connections of these items are rigid, like an intravenous syringe.
As a consequence mages dislike using these items themselves and prefer the lower-level method of making spells they fully control (i.e. the first option).
A day is roughly the period it takes for the "runic syringe" to penetrate into the "magic tank" of the user through the user. That's why it takes nearly a day to attune to a new runic item. And the reason people couldn't just switch runic rings to choose on the spot get a different magic action.
While you claim
Most normal humans don't use their magic anyways so reserving all of it isn't a problem.
and it's a normal position, from a wizard point of view, as soon as runic items get into the market, you will find many people wanting to hold more than three at once.
[of those which can afford them, of course]
 Most laymen would be unable to cope with concepts like letting the magic flow from them.