I don't understand the last clause. Yes, Google uses "dirty energy" in many data centers because they're connected to grids without direct "clean energy" feeds. But electricity is near-perfectly fungible. If you care about "clean" energy at all, your goal should be increasing the supply anywhere -- what's the difference to an environmentalist if that 50MW goes to a Google-owned data center or just a local housing development? How does it make it "less good" of Google to have built those turbines?
No, that's a very commonly stated point, and it's completely wrong from the perspective of someone trying to reduce carbon output.
Solar usually gets this criticism more than wind, so let's use that: the argument goes that you need enough power generating capacity to handle peak usage, but of course solar only works in the day. So if your peak usage happens at night (it doesn't, but let's assume) then the solar panels have "displaced zero power plants".
But who cares? If you have them built, then during the day the solar plants are operating and generating (much cheaper, looking only at marginal costs) power. And the coal and gas plants, by virtue of having higher marginal costs, are idle. And at night the fossil plants fire back up and do what they were doing before. So the net effect, assuming excess solar capacity, is that carbon output has been reduced by a factor of two (actually a bit more than two in practice).
And of course then the market will accomodate, such that "day power" being available in higher quantity and with higher excess capacity becomes cheaper and "night power" more expensive. So the legacy coal plants will probably still even make money as they're selling into a "premium" market.
So... how is solar (or wind) a toy in that scenario?
There is an interesting factoid that hasn't popped out of this discussion, Google is a power company . They did this for a number of reasons, but one of the benefits is that they get to play in the slush pond of kilowatts. Energy companies 'trade' energy (because as ajross points out it is sometimes in the wrong place at the wrong time) kind of like Internet providers 'trade' bandwidth. As licensed energy provider Google and trade wind kilowatts in Iowa for coal kilowatts in Georgia. Its a great way to 'hack' the fact that transmission costs and land costs make it hard to build renewables near your data center.