One of my players is contemplating switching away from ranger because of the stiff "penalties" the at-will attacks seem to impose. Let's look at a few items off this 4th level Elf Archer-Ranger's character sheet.
1) Ranged Basic Attack (Longbow). +10 vs. AC for 1d10+7 damage.
2) Careful Attack (Longbow). +12 vs. AC for 1d10+2 damage.
3) Twin Strike (Longbow). +10 vs. AC for 1d10+2 damage, twice.
Average damage with attack 1 is 5.5 + 7 = 12.5
Average damage with attack 2 is (5.5 + 2) * 1.1 (hits 10% more than 1) = 8.25
Average damage with attack 3 is (5.5 + 2) * 1.5 (assume he hits 66% of the time) = 12.45
Ultimately, niether class at-will attack is better than a basic attack. This is not the case with any other class. These at-wills have any other effect than damage, either, so there's also no benefit there.
Was this really by design? It seems so gimped.
As others have pointed out your maths needs to take better account of chance to hit in all cases.
True strike does more damage than a basic attack in all circumstances unless you get a very high dex or additional damage on basic attacks from items.
ignoring hunters quarry
With a 50% chance to hit and +2 from sources other than dex
twin strike does 7.5 damage/round
a basic attack passes that with +8 from dex or other sources that only affect basic attacks (e.g. the bracers that give +2)
That seems to hold for any chance to hit.
Of course if you get another +1 from magic that changes things again and you need +9 from dex etc for a basic attack to average more damage.
Including hunters quarry (at d6) with a 50% chance to hit, +5 from dex and +2 from other sources
twin strike does an average of 10.125 hp/round, a basic attack does 8.
Twin strike maintains the lead until you get +9 from dex.
if you take the feat to give +d8 from HQ then you need +10 from dex to get the lead back.
Even when the basic attack does more damage (and I suspect that changes in the higher tiers with more damage from HQ makes a difference there) twin strike looks worth it as a chance to take out 2 minions a round.
Note that I'd recommend picking the at will that lets you shift a space and then shoot instead of careful attack but twin strike is a given.