busman76, the sales rep was just giving you information based on what is common shock knowledge.
OE suspension shocks are valved to work with the springs that they come with, from the factory, taking into account both the spring height and rate and the shocks damp settings. Anytime you replace an OE spring with an aftermarket 'lowering' spring, it does change how the shock is going to handle that spring (keeping in mind that OE shocks are valved to OE springs). An after market lowering spring will reduce shock travel and cause pre-mature wear, as Tom@EAS mentioned also.
We are just here to give helpful information, we will not result to lying to our customers to sell.