As I've pointed out in numerous other threads, if we take the existence & ubiquity of D&D magic as a given, and then spin out all the logical conclusions from that, a fantasy setting wouldn't look anything like medieval Europe (no castles, for instance).
And as I've pointed out in numerous threads, that's is for most levels of the ubiquity of magic assumed by D&D wrong. What is true is that castles would not look like ruined Edwardian castles or other warm climate castles with large open courtyards and accessible parapets. But this does not cover the full range of castle construction techniques available to medieval castle builders, as a perusal of castles constructed in colder climates such as Eastern Europe reveals. What is also true is that to the extent magic was ubiquitous, magical defenses would likewise be ubiquitous, and the more pervasive your magic is, the more ubiquitous these defenses would be.
Likewise, as I've also pointed out, no D&D setting is uniformly medieval and everyone I'm aware of incorporates tropes and technologies up to and including the 19th century. So there is no need to defend a medieval culture anyway, simply to have a believable fantasy one.
The question is not whether a particular sort of magic impacts society, but how much it impacts it. A Lyre of Building not only impacts society far more than Fly, if both exists it vastly outweighs the argument that Fly would render castle construction obsolete by making it vastly cheaper to build castles. It's the existence of the Lyre of Building that you have to be really careful about, not obvious things like Fly. Above and beyond that, this mainly impacts the campaign when the players exercise their freedom to discover X shouldn't be a thing given what they can do. So it's primarily that that the skilled DM of a simulationist inclination has to be on the watch for. "What if it falls into the PCs hands will totally transform society beyond what I've already accounted for and accepted?" is the question. This feat is the answer; not Fly or Invisibility, which have more mundane sorts of counters.
My wizards don't use magic spells when mundane items will suffice. ESPECIALLY the ones with fewer spells or who multiclass.
Sure. But retrieving one rock every round so that the slinger can improvise sling stones to do 1d4 damage suggests a spellcaster that isn't very much up to his job. At the very least, he'd have been much better off with Craft Wand rather than this feat, and craft a few simply utility wands. A 1st level wand of Magic Missile or Burning Hands doesn't cost very much, and works better than what you are describing as some of the best utility this feat can be put to.
But that brings me back to my prior point: why waste spell slots when you don't have to?
What sort of spell are we wasting? An Unseen Servant lasts even 5th level character 5 hours, quite long enough for a foray into a dungeon or a major portion of a day's wilderness journey. Considering the vast utility of this spell as a problem solving device, there is little reason to not have one in a slot, or if not a prepared caster to have it as a known spell. It's one of the best utility spells in the game, and in the course of its duration more likely to be an aid to the caster than the feat is - which also forces him to take up a slot.
If you sat in with our group, a typical D&D combat encounter might have each arcane caster expending 0-3 spells, and using other methods to contribute. Likewise, the divine casters only expend lots of spells when there's healing to be done.* Ideally, only enough magic is spent to tip the encounter in the party's favor, not one thaum more.
Wise. But hardly proof of this feat's utility, and barely proof of any of the other reserve feats utility, since Craft Wand would probably get you more magical resources with only slightly higher expenditures - and that's assuming you are not playing with a DM that allows fungible wealth and magic shops (which, being 'old school', I don't).
...which is why, coincidentally, we don't see the "15 minute workday" in our campaigns. Spellcasters typiclally have enough spells to last 'till it's time to camp for the evening.
Again, even a fairly low level caster could have Unseen Servant up for an hours long workday, prep a few scrolls with similar utility spells, or procure or craft a wand. A single wand of Mage Hand or Unseen Servant would last most mages an entire campaign, and see more use than this feat would. If they only flicked the wand when they would have used this feat to accomplish some tangible and important goal (and not pilfering a coppers worth of apples from some hapless greenseller), they'd never run out of charges.
