Look, I see why people might think that discretionary trading has it's advantages, and a lot of the time I would actually tend to agree, however it's in execution that you have to introduce a systemic element that takes you away from a purely discretionary strategy. For instance; you may have an idea that came about by discretionary means, but you have no shot of executing it without introducing systemic methods that will ultimately alter the way you conduct the trade. Liquidity is a very big issue especially if you're trading at volumes. Imagine if funds adopted fully discretionary methods, they would get screwed over in orderflow every time.
To be honest, I like discretionary trading a lot. I've been doing it a lot even at big volume on my platform. It's funny, when someone is presented with infinite liquidity you'd assume they trade systematically only and at high frequency, but most of our traders (albeit we're still in our alpha with paper trading) actually trade with discretion.