Trying to ride something like a mallet as a flat (say on descents) never works particularly well in my experience, even less so if you’re wearing a relatively stiff SPD shoe with a whacking great slippery metal cleat right where you don’t want it. You just slip around on the pedal and can’t get any decent control. Mallets are particularly bad for this as the cleat tends to roll over and spit your foot off the pedal (they are great pedals though!).
Either commit to flats or or cleats, if there was a middle ground that worked everyone would be using it.
EDIT: I like mallets a lot – they’re very easy to get out of (and more importantly, to clip back in to) and, having loads of float in the cleat means you can move your foot around on the pedal a fair bit before settling, when the pins hold it in solidly in place (a bit like a flat).