If you're using SPDs on road shoes that don't have a recessed area, then it's far worse than SPD-SL, since you're walking on a skinny piece of metal instead of a wide piece of plastic with rubber nubs.

But if the SPD is in an MTB shoe with a proper cleat recession, the shoe's sole should mostly be what you're walking on, not the surface of the cleat. In some shoes the cleat will intermittently scrape the ground a little and make noise, but it shouldn't disrupt what the shoe's sole is doing.