as for exemptions, i think someone else covered it, but look at line 6 (and box 6d, specifically), and line 42 on a 1040. you get $3650 off your taxable income. not the same as an allowance, but similar. at top rates that's about $20 a week, give or take.
and yes, it's not exactly the same thing, but it is similar.
frankly i think if we're going to do this sort of welfare that system does make sense. it's kinda like the prebate system the fairtax would use. everyone gets it regardless. there's a lot more money running through the system but a lot less overhead.