This was one of the OG topics on Letsrun:
http://www.letsrun.com/forum/flat_read.php?board=1&id=102856&thread=102856
Minimalism was a thing here long before it went mainstream. Check out some of Trackhead's old threads. He talked about putting a Vibram sole (before the five fingers existed) on a pair of Kennedy spikes.
For what it's worth, I used to train exclusively in flats. I eventually worked one pair of cushioned trainers into my rotation, which I'd wear mostly on recovery days, especially if I'd done a workout in spikes the day before. Over time, I've found myself wearing trainers for a greater and greater percentage of my weekly mileage. Partly I think I'm getting old, but I also think that minimalism had led to much more natural trainers with lower drops. The Saucony Freedom is one of my current favorites, and although it has great cushioning, I feel like it runs almost like the Hyperspeeds that used to be my favorite.