I couldn't be bothered reading what everyone else said, so here's my stance on it.
I had a 100mm hardtail for racing. It was good for most XC trails. Now I have a 100mm bouncer. It's good, I ride it around my local stuff all winter. But it's a very light and steep XC race frame, and not ideal for most of my day to day riding.
My 140mm forked hardtail is, in essence, a full sus replacement. I don't want to spend money on shocks, bushings and pivots. I bought a burly frame because I want to ride DH tracks on it, freeride parks, dirt jumps as well as ride all day XC rides. I didn't want it to break on any of that, so I got a tough-as-old-boots frame.
This is designed around a 130-150mm fork. So that's what I put on there. For most of my local trails 100mm is enough, so I do sometimes run it that low, but when I go to Laggan or similar (which is very regularly) I like being high up (I don't get this "low front end" business that's going around on DH/FR bikes) to get a position I like over the techie bits, so I almost always run it at full travel. I also ride in the Peak and Lake district a lot and like to have a fair whack of travel to soak up the rocks. As for out back, your legs are better at suspension than your arms so it just soaks everything up fine.
I'm just about as fast on the hardtail as I was on my old full bouncer, but don't have to worry about maintenance. It's great. I could well do a lot of the freeride stuff with 100mm- big drops and things don't take much travel- but for the techie DH tracks (like Dunkeld) and XC trails I like having more give.