While I can't defend the optimality of today's hodge-podge of deployment incentives and subsidies and loan guarantees, there's actually strong reason to think that establishing one, uniform carbon price is not the most cost-effective way to spur renewable and other low-carbon energy technologies forward.
Each technology is at a different state of maturity and has a different price gap between it's current costs and those of very mature, entrenched fossil fuel competitors. For example, while solar PV may be 30 cents/KWh, solar thermal may be closer to 15-20 cents/KWh and wind power may be more like 10-12 cents/KWh. And beyond that, each competes against different fossil competitors, with solar PV on end-use rooftops competing against retail rates, and central station plants competing against gas peaking plants (for the most part) and wind competing more against off-peak gas-fired power from combined cycle plants with lower costs.
So one uniform carbon price as the only incentive for deployment would create an incentive too high for some technologies and too low for others, effectively stranding these techs. And since many of these emerging clean energy technologies are expensive today, yet are coming down steadily in price as incremental innovation, economies of scale, and learning effects are captured, it would be costlier in the end to strand those emerging technologies that are higher in price than the incentive created by the uniform carbon price. Targeted incentives for each class of clean technology may actually be optimal, especially if designed explicitly to reward innovation and price declines, not simply create permanently subsidized industries (see our recent Post-Partisan Power report for more on reforming our mess of existing deployment subsidies).
Furthermore, the final objective of US climate (and clean energy export policy) should be to make clean energy cheap enough to power the vast and growing demand for energy in the developing world. The IEA projects that between now and 2030, on a BAU course, developing (non-OECD) countries will be responsible for 97% of emissions growth in that period, with emissions growing enough to entirely outweigh a complete elimination of developed (OECD) nation emissions.
These developing countries are unlikely and for the most part incapable of sustaining high carbon prices or high permanent subsidies for clean energy. Remember that for 2.4 billion people in the developing world, roughly one-third of the entire global population, who still rely predominately on dung and wood and other primitive biomass for their energy needs, all modern energy is now too expensive. Promoting a shift to even more expensive clean energy is unlikely to be a winning formula. So why would we want to focus on creating an artificial market environment in the United States or other OECD nations that only supports clean energy by making fossil fuel competitors artificially expensive through carbon price, or makes clean energy artificially cheaper through permanent subsidies -- market conditions that won't be replicated in the developing world where the vast bulk of energy demand and emissions growth pressures will come from?
The only thing to do is to focus on making clean energy cheaper in real terms, through steady innovation. Smart, optimized, and disciplined incentives for deployment and scale-up of emerging clean energy sources can and must play a key role in that effort.