I just responded to someone in the forum who had put together an experimental Survival build with a talent calculator, without Mortal Shots, and was requesting opinions on it as well as wanting to know why MS was really necessary.
I feel that this is an important issue. Although I maintain that Survival is fundamentally about thinking outside the box, I've learned that there are certain situations where it's actually important to think very much inside the box, and that this is one of those scenarios. My forum response follows.
The only thing that is really going to teach you anything reliably is your own experience. I used to spec very melee heavy (in terms of Surv) without MS, and do a number of other crazy things. (I'd never do it for serious use, but EW, Imp Hawk, Master Tac, and Rapid Fire all proccing at once is something every Hunter needs to experience once, IMHO ;-))
People here in the forum called me a scrub, a newb, and a munter, but I didn't listen. Then however, I started dying. A LOT. I'd put 100g away towards an epic mount, and ended up having to spend probably 75 of it on repair bills during my first three days in Hellfire Peninsula. I got rolled that hard the first time I went to the Hellfire Ramparts that it almost hurt behind the keyboard. My results in battlegrounds were sometimes good, also, but if I was honest they weren't as consistent as I might have liked.
So I went back and thought about it. I respecced, first straight Marks, (which, with all love and respect to my Marks brethren, wasn't for me, personally) and then at the time to 0/20/35 or so. For admittedly a brief period, after doing this I topped the kill chart in AB once, which was something I'd never done before, and my ranged damage was up a heap in 5 man instances, as well.
In short, there are concrete reasons why a lot of the people in here advocate the things they do; however, sometimes we use the degree of vitriol that their advice is often served with as an excuse not to listen to it. I'd actually recommend that you don't listen initially...go and spend 200-250g or so on respecs, experience everything in the three trees that you can...you'll end up a far better Hunter for it.
However, you'll also learn why people say that you really should take Mortal Shots.