Ladies and gentlemen, my first car ever was my grandmother's old 1986 Ford Taurus. It had a digital speedometer and a pretty kicking tape deck. I even got the sample cassette tape that came with the car. I spent some of the best years of my life cruising the Garden State while listening to Neil Diamond's Heartlight and the theme to Cats. (Before you judge, most of you were listening to Sugar Ray and the theme to Friends instead at the time.)
What in the hell does that have to deal with shadow priesting? The car was the right fit for me a decade ago, but it's just not right for me anymore. In Warcraft, just as in life, the right choices change over time. I mean, what the hell am I going to do without an iPod connection in my car? And Ford Tauruses -- do they still even make those?
When I applied for a new raid team a few months ago, I went to my class trainer and put together a pretty good build for my shadow priest. It worked great then for the content I was working on. After re-examining my build, I realize that times have changed, and so should my talent tree.
Psychic Horror was a great ability during Trial of the Grand Crusader progression. It's useless in Icecrown. And it's not the only talent that has outlived its usefulness. Shifting around a few points can have a very noticeable effect on your DPS, mana regen, and survivability.
The price to respec varies -- it could cost you as much as 50 gold depending on how many times you've done it recently. Still, when you compare that to the amount you spend twice that on a single epic gem and even more on high-end enchants, respecing gives you one of the best returns on investment in the game.