The Steam Machine is actually a great prospect — we're just looking at it wrong
(www.xda-developers.com)
And? The problem is steam not being able to sell the console at a loss. The hypothetical call centre is welcome to sell the vouchers. They've still paid full price for the machine. This makes it a much more enticing prospect for people that actually want to game on it though, and so long as the bundled voucher eventually gets used, why would it matter that it's not the person that bought the hardware?
ETA: Also, if the voucher doesn't get used at all, Valve win entirely