Okay, I'm not understanding your position. How are you suggesting that we define the "worth" of an employee's productivity then?
Via the markets. How much is someone willing to pay you for your skill or ability or dumb labour? To me, that's the only definition of "value" that means anything. It's what makes Banksy and Damien Hirst's art "worth" more than an art-school graduate, even if the effort, thought and intricacy of both is the same.
For the purposes of a minimum wage, this isn't really relevant - by definition, it's subverting this market based approach in favour of a redistribution of wealth, and that's fine. By and large I'm in favour of it from a purely macro sense. I just find it a little odd when people talk about how awful employers are for giving low wages or even wanting to pay lower wages were there not a minimum wage, as if they should be expected to perform what is in effect an act of charity with their company's profits. The whole point is that this labour isn't worth as much as the minimum wage, otherwise it'd already be receiving it.
In other words, I think a robust defence of the minimum wage should come from the position of economic benefits and possibly less social tension via the possible reduction in income inequality, rather than a sort of weird emotive argument about how labour is intrinsically worth more because they're humans etc.
Edit: to clarify and as a response to The Technomancer above, when you "assign" value to labour, it ceases to really mean "value", surely? You can do it with a fiat currency because, assuming there's reason to have faith in a currency, when it's determined that $1 is now worth $2 instead, you can then use that to buy $2 - or what used to be $2 - of product with it. That's not the case with labour. You can assign it the value of $10.10 if you want, but that doesn't mean an employer actually gets $10.10 worth of productivity.