I'm in two minds about this. Yes, I think purchasing currency directly can lead to quicker growth.
However, the reason I like GPU mining, even if you don't have a 4 GPU rig, is that you own the means of production. You generate more coins. And, if you are a gamer like me, your hobby pays for itself.
I do think that if you are GPU mining, you have to keep informed about trends in terms of what is profitable to mine and what is not. And, I also think it would be worthwhile doing speculative mining, where you mine a currency that is not worth much at the moment, because the difficulty will be lower and you will mine far more coins. If it ever suddenly increases in value, you can make a killing for relatively little investment.