I look at protein as if it's name is "meal replacement powder"...
When gaining weight, most websites/training programmes/magazines recommend 5-6 meals per day.. This is a very good start for gaining weight, as most people won't eat enough during just breakfast lunch and dinner (3 meals) to overload their bodies with excess material to build a dramatic amount of mass..

Now I would never replace a real food meal with protein any day of the week. You can't beat a good solid meal for nutrition & protein (run a search on yahoo or something for foods rich in protein).

But for most people, it's not practical to have 5-6 meals prepared every day.. Especially if you work or go to school.. And that's where protein comes in.. You can have breakfast, and then around 10:30 -11am have a protein shake.. then lunch, protein around 4pm.. you get the picture..
It's really their just to give you that extra intake that you otherwise wouldn't get.. It's also a lot easier to get 40 grams of protein from a drink than from eating a steak.. who could handle 3 or 4 steaks every day..
So make sure you've got your standard meals up to scratch.. never replace a proper meal with protein unless you just don't have time for a good meal. Once you've got your normal meals down and still aren't gaining the mass you hope for, add a dose or 2 of protein powder into your day..

A few things to be aware of with protein.
*It's not the magic answer to building mass,
*No matter what anyone says, its not as good as a real high protein meal, and these should be your FIRST priority.
*And if you are not completing a hard work out in the first place, extra protein is a waste of time & money.

BUT, added to an already solid meal & workout routine, protein supplements can play a BIG part in gaining extra weight and muscle mass....