You can not visit Peru without going to Machu Picchu. Or do you really want to tell your friends you didn't see the only place in South America they actually know? So you are kind of forced to go.
But sometimes, being forced to do something is not all that bad, and this is one of those occasions. Doing the trek to Machu Picchu was one of the best experiences on my trip to South America, to recommend to anyone, and I'm really glad i got forced to my luck.
A big plus: the region around Salkantay mountain looks like Switzerland, and it's always good to find a piece of home far away!.