Have you found a complete rifle that is EXACTLY the way you want it? If so, and you have the money to buy it outright, then there you go.
Personally, I'm a big believer in building. If you like this style or twist rate of barrel, but that type of stock, for example, then building it is the way to go. Especially if you're like me, who has to build their guns one part per payday. Why spend money on a complete rifle that has parts that you don't want and are just going to change out anyway. Build it the way you want from the start. Unless of course, you're one of those Colt collector freaks who will leave it in the box and never shoot it, in hopes of selling it for a bazillion dollars one day.
Do you like onions on your burger? I don't, that why I go to Burger King, so I can have it my way.