The biggest reasons for the whole subsidy/tariff plan is two-fold:
- renewables (of any type) reduces dependence on imported energy sources (oil, coal, whatever)
- most renewables cost a LOT when first introduced into a market.
The second part (at least insofar as solar) is changing rapidly. Thanks to a glut of PV panels on the market, solar is costing manufacturers around ~$1.50 USD / Wp (on average). It used to cost $5 USD per watt, and in the early 1980's, the figures reached horrendous heights. By end-of-year, PV* production should cost around $0.75 - $1.45 USD / Wp, depending on quality (the lower end being thin-film, the upper mono/polycrystal silicon - you get what you pay for, though).
* For everyone else: PV = Photovoltaic :)
Now note that these costs are rapidly approaching the cost of oil-produced energy, and IIRC is below the cost of wind-generation, which is also dropping. As economies of scale kick in, things get even cheaper. Currently, there are but a handful of facilities capable of producing at least 500MW worth of panels per year - a couple in the EU, one in the Western Hemisphere (come Q3 - and that corp is German-owned), and I think one or two in Asia. In the next five years, I suspect that the total number of factories at this level of output will double, perhaps triple (though the majority will likely be in Asia).
At that point, PV solar should be cheap enough to install that tariffs (the official term for "subsidy" in most countries) will be largely unnecessary in most nations (depending on energy costs, etc).
Now for some irony. The Spanish PV guys are whining, but even now, Germany is beginning to drop theirs. Now for the funny part - the big German solar manufacturers: Q-Cells and SolarWorld AG stand out -- they are #2 and #3 in the world, respectively -- really aren't seeing problems with Merkel cutting back the solar tariff in their own stomping grounds (now mind you, the German PV market is pretty saturated, but still...)
Long story short - the big boys aren't seeing too much problem with the state of tariffs. Sure, they'd love to see them - it only boosts sales. OTOH, they've figured it into their projections and can survive regardless, given the momentum that PV solar has gained, and is continuing to gain.
Not sure about wind - I don't hang around that segment ( I do however hang around the Solar bits of the renewables industry... far too much, methinks :) ).