There is a lot of stuff overblown right now, that you need a 64 degree head angle, that you need 7" of travel, that you need 2.8" tires, and on and on.
Lots of people are over-biked, a lot of the time. I'm over-biked a lot of the time. I wasn't yesterday during some of my ride when I was doing some ass-way-behind-the-seat steep chutes and some high speed DH (4500 vertical), but most of the TIME on that ride, I was still over-biked. Fast forward to today, took my XC bike to the more local trails in the city and it's a blast. I can hit turns and jumps with more speed. No one is losing XC races because their HTA is too steep or because they can't ride downhill fast. With a 29er wheel, my bike is in no danger of endo-ing unless I do something drastically stupid.
There is a lot of fun to getting out there and having the right bike for the trails, you can maximize your speed and have a lot of fun dodging trees at mach5.
IMO, it's not as fun if you ONLY own a very-XC bike, unless you live somewhere that naturally limits what you can do, such as out in the midwest and some places that just don't have big terrain. But on the other hand, it's real fun to get out on a trail and have the right tool for the job. With more experience and riding, you find that you can ride such a bike very fast, land on the transitions, know when to loft, etc.
At the higher levels of XC racing, you realize that these guys ride their XC bikes harder than most intermediate "all mountain" or "enduro" riders. At these levels, the riders are usually multi-discipline too, they often ride enduro and DH as well as XC, for the variety. Some of these guys have more bikes to choose from, some of them don't and ride the hell out of their XC bikes. Went and rode Sedona right after the Whiskey 50 with some fellow expert riders from my state. Even advanced riders wouldn't ride all the stuff that we rode. Would I have liked to have my bigger bike for it? Sure. But realistically, bigger tires and fork would have gotten me 80% of the way there and allowed me to have a big boost in security. So this year I'll bring some extra tires and maybe my extra fork or switch it out for that trip. That said, we did just fine coming down stuff like Highline and the Hogs on our "XC" bikes. There are things you can do for shorter travel bikes like a Pivot 429 Trail with only 120mm of travel to really boost the technical ability, like a slightly bigger fork, some bigger tires, and so on. On the same token, you can turn it into a dog with big + sized tires, wide rims, heavy fork, and so on. Kind of nice with something like that to have some rim and tire options to go back and forth.
My main point is, unless you are riding stuff all day long that is the equal of chairlift and full on DH-bike terrain, no, you don't need an AM bike and you are probably over-biked with one. That isn't necessarily a hugely bad thing, I love to blast off obstacles and ram through rock gardens, but on the other hand, I also appreciate a fast light XC bike, which can also be a blast, taking corners faster, allowing you to hop off stuff that you just can't with your bigger bike. These bikes can be fun and you have to figure out if your terrain and riding style really needs all that extra travel and beef. If it doesn't, then you are just lugging around extra pounds and energy-absorbing suspension.