[JRTJH]

The Keystone Owner's Manual contains the following:

• Weather and atmospheric conditions will cause rubber to deteriorate over time. Inspect seals around slide-rooms regularly and replace at the first sign of a problem.
• Periodically, during the normal course of operation, Slide-out systems and rooms will require adjustment. This is covered under warranty the first ninety (90) days after original retail sale.
• Slide-out rooms should be stored retracted.
• Keep mechanical components clean of road debris, salt, etc using a mild soap and water.
• Rooms that will be extended for long periods of time should be actuated 1-2 times a week.
• Failure to follow these basic steps can lead to intermittent operation of the slide-out system, improper sealing, personal injury and damage to the RV which is not warrantable.

I took the liberty of bolding two of the steps. While there are many who don't follow the owner's manual and have had no problems, the above is Keystone's recommendation. YMMV

As for "shortening the life of the components by using them, most owners find that using them keeps rust, corrosion and debris out of the system and actually lengthens the usable life of the components. Think about things like corrosion on the contacts in the electric motor, dirt/cobwebs building up on the seals and seals drying out/sticking to the sidewalls of the slide as examples of what could happen if the slides are not "exercised" regularly.