Here's the problem with this method:
Suppose the ride height problem is the result of a weaker spring on one side. The spring has a 'spring constant' associated with it that determines the force it exerts for a certain distance of compression. This is also called the spring 'rate'. Two springs with different rates will exert the same force at some different distance of compression. What your tech has done is shimmed the weak spring to a distance where it exerts the same force as the other one - at rest. However, the two springs STILL have different rates, and will respond differently to compression forces on the suspension. The result is that one side of the car will behave differently than the other, which in the extreme can be quite unsettling. The proper repair is of course to replace the spring.
Note that if a different suspension component is at fault, a similar reasoning applies, and any solution which simply addresses the static appearance will fail the dynamic test. The tech's solution was cheap, but what he shortcutted was the diagnosis time. Only one part is probably failing and only that part needs to be replaced, anyway. The spring mount is unlikely to be the true culprit.