Crafting inserters instead of requesting them is a poor counter-example. They take 0.5 second and use basic, very common materials. For example, I typically carry at least 100 gears and one full stack of circuits plus a few hundred iron and a bit of copper, so I can easily make whatever inserters I need on the fly. But half the reason I do so is that there are more items I use frequently than there are request slots available, and a set of half dozen basic materials allows construction of at least two dozen types of items, from inserters to splitters to assemblers. But if I could at-will request some number of whatever I like to restock, I wouldn't need to carry raw materials at all, and could reclaim a fair amount of inventory space for carrying stuff to build with instead of stuff to craft the stuff to build with.
It would be an even greater benefit for more complex or slower to craft items. For instance, who can spare the inventory space to carry around enough engine units to build locomotives at will, or electric engine units to build small pumps? And modules take forever to craft, even if you do carry the advanced circuits around with you. Crafting electric furnaces would require stone bricks, something used only for those, oil refineries, and walls, all of which take plenty of time and/or lots of materials to craft in useful quantities, so there's no reason to carry bricks - but I don't want to be requesting furnaces and refineries all the time either. And I don't want to carry around three stacks of efficiency modules all the time - only when I'm picking up 50 electric miners to set up a new outpost. Then there's roboports, large fleets of construction bots, and enough solar panels and accumulators to build four new roboport-green-zone-sized power farms. There are plenty of other examples of things that are a nuisance to craft yourself and/or are infrequently used but aren't necessarily things that some people want to carry around all the time.
And what are you supposed to do when you have all 20 slots filled with items you do want to be constantly supplied with? Half a stack of each inserter type. A stack of three different types of ammo for your three weapons. Grenades, poison, slow, destroyer, and distractor capsules. Lights, electric poles, substations, blue and red belts, undergrounds, splitters. A stack of ground pipe and normal pipe. Rail signals, straight and curved tracks. Walls. Laser turrets. Repair packs. Level 3 assemblers. A stack of iron, copper, steel, gears, circuits, maybe adv. circuits. That's 32 items even if I leave out the six crafting materials at the end (and I didn't list yellow belt stuff either). It all adds up, and I have filled all 20 slots plenty of times.
Currently your only recourse is to remove one of your requests and configure a new one, remembering that it's the number you want to have total, not the number you want delivered in addition to that. Then you have to wait, and watch your inventory to see when you have the full amount. Then you have to remember to clear the request before you start using the item, or you'll end up having more delivered, leaving you with extra taking up an inventory slot! And then you have to set up the original request that occupied that slot. All in all, a very slow and cumbersome process for something that should be quick and simple. I mean, it's easier to drop a requester chest and configure it, get back to work for a while, then come scoop up the chest along with its contents! And if a roundabout solution like that is easier than the "proper way", then something's wrong.
The character logistics requester slots are perfect for things you want to always be supplied with, but they really aren't the proper type of interface for one-time requests. I'd suggest a different interface than the OP's idea, though. The inventory screen is already wider than I'd like!
All you really need is a button below those "permanent" request slots. Have it open up a window just like when you're configuring a requester chest, with a "deliver" button which closes the window and sends the order to the bots. (Having that window list stored amounts the way he suggested would be nice, but not absolutely required.) Unlike permanent requests, if there aren't enough materials presently available, you'd probably only receive what was available at the time of the request (which is fine, and would be something to keep in mind when setting up smart inserter limits on your passive provider chests), unless something hidden under the surface tracks requests even without a persistent requester slot.
Then you can go about your business. You don't have to keep track of anything. Request other items, finish preparing for your trip, or build what you do have the supplies for. And you can start building or crafting with the requested items as they arrive, since doing so won't result in receiving more than you wanted.
That's the use case this suggestion applies to, and I think it's a really good one (I was planning to suggest it myself at some point).
TL;DR:
1. Good use cases are bulk deliveries, atypical requests, or items slow or impossible to craft manually, or which take specialized / rarely used components.
2. This kind of system is better than permanent requester slots for stuff you don't want to always be carrying around without fail, or for when you know exactly how many you need and don't want extras left over to clutter up your inventory once you've used those up.
3. The 20 character request slots can fill up easier than you might think. Then new requests (even one-time requests that you'll clear immediately after) become very cumbersome, requiring several steps, some of which must be performed after the delivery arrives. When carrying around a spare requester chest to use just for making one-time requests is the "easy method", something's wrong.
