At lower levels you craft a trinket of unlimited charges of magic missile, fairly cheap, but you still can cast one per standart action.
Technically works better than shooting arrows if you are not in close combat, but otherwise a full round of attacks does more damage.
At high levels you may craft your trinket of some better spell, say, ice storm, not so cheap but still can manage to fire it once per round.
Problem is your foes are probably strong or smart enough to endure a couple rounds to get to you and engage.
And that's besides the fact the DM may copy your tactics on the grounds that the NPCs are not as stupid as on machine AI.
On epic levels however, the fights may take place in a single round, but that round will take forever to resolve.
There's such thing as too many rolls to make, and a lot will be opposed rolls alone.
At this point you can't really count on magic items let alone craft them.
But this kind of game isn't the norm in d&d as you mention, the lower levels are more down to earth naturally.
With 4e they wanted to cease worrying about charges, slots and activation and changed it to abilities that can be used per quest or per round, thus much easier to track.
So now you have unlimited magic missiles that might still not be too different from firing arrows, which you won't track as well and just assume you won't empty your quiver in one fight.