Variety keeps people motivated. Variety keeps people from being burnt out. Variety... it really makes life interesting.
Yesterday, I had one of the most busiest days, and I only made it because I added that variety in that day. I packed most of my things, watched a couple episodes on netflix, got on the computer for a bit... and played games... it was wonderful.
Adding variety to your workout does the same thing. It adds to it, keeping runners from being burnt out and adding that 'spice' to make things interesting. When you live, live with variety and don't stop with the same old things because that's when you start to stagnate. Change it up, do different things... and most importantly, have fun.