Part of practicing a "standard" opening is learning how to react to moves that diverge from the main lines. When playing against real people, they're sure to throw in an odd move or two, especially at lower levels. It usually only means that they're playing themselves into a worse position, so you should definitely know how to exploit this. If you know how, you'll come out on top every time.
If you just want to play main lines, play against a more difficult computer or more experienced human players. But if you keep losing such games, then it's just as well that you practice tactics regardless of your choice of opening.
I've only been on chess.com for a day or so, and I really like the chess computer feature, but I keep having problems when playing standard book opennings against it. Basically, the chess computer doesn't follow with the mainline openning, and therefore I can't really follow the mainline either for obvious reasons, so i'm having to think of alternatives instead.
The problem I find with this is that I can't therefore practice with standard opennings...