If you could convince someone to sell you a box full of Block Erupters at about $1 each, then they'd be great. At about $5/each, they'd be so-so (and still about to be obsoleted, cost-wise). Anything more than that, and they're just not worth the cost anymore.
6 of USB .333 asics at $60 per pack would be same as 1 USB antminer. Just not nearly as efficient.
What happens when they begin developing sha256 alt coins ?
Or scrypt asics get developed ?
hmmmm, not to far down the path, even if only real purpose is to put something out there for old hardware to do.
GPU's with the high expense, low availability, Heat output and power use, really better for the game industry.
I can still process 20,000 credit card transactions per min on 1992 thru 1998 servers PPRO, PIII xeons quads, hex, and octs. Not sayin I want to - Power costs killed those off.
So I think Asics will be around awhile, for the Po folk to hack on. Always a way...