For me, since I was a toeclip user, I would not ride my bike without some type of clip. So, there was no point in single sided SPDs (one can't drag unused toeclips around).

For SPDs (and others), there are adapters. The Winwood Decksters are available for putting a platform on SPD pedals.
I snagged a pair of Winwood Instep Toeclip adapters (somewhat rare now). I didn't think they were that comfortable to use, but I have them just in case I choose to go without biking shoes.