Fun thread. I've spent *way* too much of my practice time working on double kick just because I want to be good. In real life, even on metal songs, people don't generally want much of it ("make it simpler, less double bass"). I'm not into super heavy metal myself, where it's just continuous 16ths, but Primus type stuff, or even Dream Theater is stuff I like, and "Portnoy style" quads sounds really cool at certain tempos where 32nds fit the bill. But I'd probably be ahead if I had spent my limited time working more on my hands.
As a tangent, I've been working on "linear singles", or "ladder" (Todd Sucherman term) and it's still defeating me after many months. It's where you alternate hands and feet like [right-hand,right-foot,left-hand,left-foot] or [right-hand,left-foot,left-hand,right-foot]. It sounds super cool at high tempos, but so unnatural compared to other drum moves. I can't break 120bpm, which I can pretty much do with only the right side of my body anyway...