I think the logic is this. With good sanitation, it can be assumed an infection won't and hasn't occurred. When purposely adding bugs, you now know for a fact that the equipment has had non-sacc yeast or bacteria in large quantities in or on it.
Most simply keep to sets of soft goods such as tubing, bottling buckets, plastic fermenters, racking canes. Glass is a little less worry some.
I think the best course is to retire some older gear to sours only and get fresh gear for your standard beers.