Linus says:
Free spells at your highest casting level means the ridiculous imbalance between full-casters and non-casters will be virtually intolerable unless they also focused on increasing the utility and options of non-casters. No one can compete with a full-caster that is creative with spells.
Limiting it with component rarity, cost, and spell slots helped keep it within reason (though the imbalance still existed). Cantrips were brought in to give them a low-power option to use freely. If that low-power option is now just a normal-power option that's also free, then I hope it's because they saw a reverse imbalance in playtesting. I hope it wasn't because their full-caster playtesters whined louder than non-caster playtesters.
Cantrip scaling is something that D&D 5e does as well, and I've found that it does not overpower the casters at all. It really depends on what they do with the other spellcasting rules that will determine if it makes them that much more powerful than the martial classes.
I would assume they aren't as powerful as spells that have a cost at all, just that they improve with levels. A possible example:
Level 1 spell does 2d6 damage to a single target - Uses a spell slot
Cantrip at level 1 does 1d6 damage to a single target - Is free
Level 3 spell does 5d6 damage in a radius - Uses a spell slot
Cantrip at level 5 (when you get level 3 spells) does 3d6 damage to a single target - Is free
I imagine that this is the sort of scaling that we will see in the game. Something to make casters better when they don't want to use a spell slot, but not enough to come anywhere near overshadowing a real spell. Obviously I don't have all of the details, but I think it's a good guess, at least as far as design philosophy is concerned.
[ +- ] Quote from Devs - RE: Cantrips
Cantrips aren't 0-level spells; they're spells you can perform all day. The damaging cantrips aren't super powerful compared to a martial's attacks in any case, but they'll still change what sorts of things you want to prepare in your various spell slots. So for instance, if you're really high level and have a cantrip that deals the same damage as a 1st-level magic missile, it might make sense to prepare utility spells in your 1st-level slots unless you have a reason to really want no-miss force damage in particular on that day.
[ +- ] Quote from Devs - RE: Spellcaster Scaling
Well the key to making a system less prone to those sorts of problems is that we need to address those problems in a balanced fashion. If you just make spellcasters weaker, that's not particularly exciting. But if you give all sorts of fun new opportunities to heighten spells and much better at-will cantrip options? That's a much better way to design a system where the casters are no longer quite as exponential in power increases (heightening being how you increase effects means you should hopefully no longer have a situation where a 20th level caster can still end the entire fight with one 4th level spell, which she can at that point do for every fight because she has dozens of spells of 4th level or higher), but it allows more opportunities and cool stuff that feels like more power, as you mention, when it's really different power, evening out the smaller turns you don't cast your big wow spell (through heightened cantrips) while lessening the number of "I win" buttons.
[ +- ] Quote from Devs - RE: Example Cantrip
If we're talking 1st-level, you're looking at roughly as many prepared spells, plus your Spell Points (maybe 4+), potentially some additional spells from other class features, and then the cantrips are significantly better than doing 1d3 damage even at 1st level. For instance, telekinetic projectile (the most damaging single target cantrip because it hits against full AC) now does 1d10 damage at 1st level, which is much more than it used to.
[ +- ] Quote from Devs - RE: Multiple Spells Per Round
You have to be very careful about how much damage a 1-action spell can do because it can be cast 3 times per round, whereas a 2-action spell is once per round.