Lowering your car using lowering springs may or may not be detrimental to your shocks / struts in itself. Running your shocks / struts anywhere within it's travel range should be OK, I've run other lowered vehicles using stock shocks / struts and lowering springs for many thousands of miles with no ill effect.
Operating your shocks / struts in a lowered state isn't what kills them. What does kill them is allowing them to bottom out, this will destroy them in a heart beat. As long as their operating anywhere within their travel allowance they should be fine, not ideal maybe, but should work fine and last a good long while.
Just be smart and watch for speed bumps, pot holes, and very rough roads. Slow down and take it easy over these kind of things so that you don't bottom out. If you watch cars travelling along a normal roadway take a look at the tire / suspension travel. There's barely any movement up and down. There's a combinarion of actions between the suspension travel, sidewall flex, and the car's movement that absorb roadway imperfections.
We ran our son's Focus on stock shocks with 2" lowering springs for 50k plus miles and never had a problem because we were very careful how the car was driven over anything that would cause a lot of suspension travel. But by the same token, I also know of several folks running the exact same setup on identical cars and they killed their shocks and struts in a very short time. But every one of those folks admitted that they wasn't careful about obsticles. - Dan