I have SPDs, and there have been several situations that have required to me to clip out quickly when I hit an unexpected root, or my wheel slid out on a berm. Every single time, my foot has come unclipped without even thinking about it. I have them set up very loosely to accomplish this, but I never have problems staying clipped in when I want to be.
It's personal preference, and I prefer to be attached. I find I slide around less, and have better power up hills. But if you prefer platforms, more power to you. Many people on the forum do.