Hmmm, I'm not sure I buy this being a big "restriction" on "casters" in general, rather on "whichever caster has to blow most of their resources on healing" - which was always an issue in 1/2/3E (unless one went the wand of CLW route). It was also why Cleric was an unpopular class (IME). 1 spell/encounter minimum at higher levels, for, say, a Wizard, who has 15+AR spells/day at L10 doesn't seem very onerous.
Yeah, but how long of real time (months or even over a year) does it take to get a wizard to level 10 in most groups?
If you had read the "did you get to level 20" thread, a lot of groups do not get that high level. Level 12 or 14 seems to be the max in many cases before life or boredom or whatever interferes and that's for the longest campaigns at each table listed on that thread, not the many many shorter ones that happen all of the time.
It's not high levels where the squeeze occurs, it's single digit levels.
It's sloughing through the mud of not being able to cast even one non-cantrip spell every single encounter because of the desire/need for mage armor and/or utility spells and/or conserving spells, and when the PC finally gets to the point where he can reliably cast two spells per encounter, the campaign often ends.
And, it's doubly difficult for the divine pure spell casters because of party healing expectations.
But think about it for a moment. Even at 10th level like you mentioned, 1 spell per encounter is easily possible. But how about 2? That's a bit less likely due to other demands. For example, the dungeon is far away. The party decides to travel there, and then grab an associated object from that location at the end of the first day, and then teleport home. They then teleport back in the next day. That's two teleports (5th level spells) per day each day they do this. Sure, the wizard gives the party the capability to do this, but at the cost of 2 of his 3 potential highest level spells. Safe, but costly. While there, they need a Comprehend Languages or some spell to get them past a hazard. Or they get into a really long difficult fight that takes up 4 spells. Suddenly, 2 spells per encounter starts becoming a little less frequent.
How about 3 spells per encounter? Those are nearly impossible to consistently achieve at 10th level.
And most of the spells are low level ones that hardly affect foes. A first level Burning Hands? White noise at level 10 most of the time. A first level Sleep? It doesn't even affect 6 HD foes usually, let alone 10 HD foes. Magic Missile? Seriously? Against 10 HD foes?
So yeah, you can claim that they have 15+ spells, but most of the lower level ones go from being combat spells to low utility spells. 1st and 2nd level spells, about 40% of the 10th level spells, are pretty darn weak compared to the competition.
So, the wizard against 10+ HD foes at level 10 is still firing off a boatload of his dinky cantrips (that take 10 to 15 rounds to take out a foe) or his light crossbow (ohh, 8 to 12 rounds) and even once in a while, he can be brave and cast Magic Missile for 10 points of damage or Scorching Ray for a possible 7 points of damage to 3 foes. Woo Hoo!!!
Sorry, but the boredom factor for players of single digit level spell casters can be quiet high, and not just for the divine casters. They either nova and accomplish quite a bit with their highest level spells, or they do double or so the damage of a cantrip burning off low levels spells, or they dink a foe.
And it takes the player 6 to 18 months of real time to get to the point where his PC can
reallly contribute once per encounter. zzzzzzzzzzzz