When I worked for a library services company, I was required to take a vacation each year of no less than 5 consecutive workdays in a row. The idea behind the forced vacation was to make sure 1) that I was actually taking some time off from work; and 2) that I wasn't creating my own little fiefdom within the organization where we couldn't survive without me.
I remember one time questioning my boss (different job) about relying too much on specific individuals for important business processes. When I asked my boss who was backing up the guy (I'll call him 'B.A. Baracus') in charge of our email system when B.A. goes on vacation, the reply was that B.A. was required to take his work laptop with him on vacation and therefore there wasn't a need to have a backup for B.A. I sat there and nodded, watched B.A. lug a 10 lbs. laptop bag over his shoulder, and then went back to my desk and made sure my resume was updated and ready to send out.
Now, granted, there were many little things that didn't get completed while I was out on vacation, and my physical in-box was stuffed full of invoices ready to be sent to accounting, but almost all of it could have been handed off to someone else if I had decided to escape from the world and become a recluse in a cabin somewhere around Pikes Peak. I'm glad that I'm appreciated at work and missed when I'm away on vacation, but I'm also happy to see that I haven't become unreplaceable. Now I can go on that winter cruise to Mexico and not even worry about checking email... that's a great feeling.