I'm in the camp that says you should have to prepare the level 0 spells. I don't really see why the Tier 1 classes need the bonus...
You do, for caster classes that have to prepare spells. That aside, the purpose is pretty clear: pure casters are weaker early and stronger late than martial classes; a 1st level wizard or sorcerer already has a pretty small 'load', so to speak, in a fairly normal 1st level party, never mind one like this. The intent of the balance changes was to make non-tier 1-2 classes more effective at higher levels, and a counterbalance to make casters have slightly more depth at low levels, aka not having to say "welp, that's my (counts on fingers) spells for the day, I'm useless until we rest."
The only arcane cantrips which retain effectiveness after the first few levels are ones which don't really have direct combat/debuffing/CC applications; prestidigitation, arcane mark, detect magic, &c. Being able to deal 1d3 damage per turn or grant +1 on saves to a party member per turn is hardly overpowering in light of what any of the melee classes (or even the warlock) can do. That's the primary point of the cantrips, basically as a substitute for what low-level wizards and sorcs normally do with their hand-crossbow or sling; they're sitting around in the back plinking things with Ray of Frost or Acid Splash instead of those while waiting for opportunities to use their real spells. The same applies to other cantrips that aren't really combat spells; they're basic utility spells and if you didn't have a source for them you'd probably just end up with a pile of wands that do the same thing for the same character to UMD on.
To put it in perspective, Ruvitsuvi does 1d3 damage with Ray of Frost. Cernan cures 1HP or does 1d8 with his crossbow. Osoero does 1d6 with Eldritch Blast. Ulric does 1d8 with his longbow. Tradok and Slork do 1d12+STR with their axes. Krag does 2d8+13 with his greatclub. Each of those is potential damage per turn. I really don't see how being able to cast a 1d3 cantrip each turn is a problem. There are three orisons which pose problems, which are easily solved with a houserule, and only one of them was more than situational (if you knew that an encounter was coming and had a set start point like a door breach you could spend a minute buffing someone with temporary HP; and I can imagine a scant handful of situations where continuously producing water could be exploitable).
Personally I'd just rule that you can't use CMW out of combat unless it's during a full period of 8-hour rest. That solves that problem, and as above, healing 1 HP per turn in combat isn't exactly worldshattering so much as being slightly more useful than standing around doing nothing much. I suppose the same should apply to Virtue and Create Water, but those are really the only ones I can see with potential for abuse, at least within SRD.
Basically, in summary, cantrips are not major factors even in RAW level 1 campaigns. They're ways for casters to not be totally nonfunctional baggage at early levels after using their spells/while hoarding their spells for the perfect moment, and it's boring as fuck to play a caster (
especially an arcane caster) at low levels for exactly that reason. All unlimited cantrips do is make it so that they have something mildly productive to do with their turns at low levels, while they'll always have better uses for their actions at high levels.
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On an unrelated note, that last elemental needs about a gentle breeze to keel over. Just waiting on a stray wind to blow by...