I've been taking several different powders (L-glutamine, BCAA, Milk Protein Concentrate, Whey Protein Isolate) for about five years and it's helped me an awful lot both in losing fat and gaining lean weight as it makes it a lot easier to get the proteins I need for fewer calories overall (as other sources are much higher in fats and carbohydrates). There's no way I would be able to work out as intensively or as often as I do now without them either. There's no harm in it, as long as it's just protein - just beware what the actual product you are consuming contains.
The only possible downside is if you don't adjust your diet and consume more fibre and fluids when you start taking the supplements, you'll feel like you've been stuffed up with a cork. I think when I first started out my colon actually solidified.
Unsurprisingly, many protein supplement vendors also sell powdered fibre.