I'd agree it doesn't make sense on a permanent basis. However, if you're looking to use the same bike for racing/training/ long rides, and also for errands or short commutes, using the adapter for the latter would be a workable solution.

Heck, I've ridden around on eggbeaters(!) without changing shoes.

But...why would you want to? Also SPDs and sometimes eggbetaers come with those dinky little test platforms that have a cleat onthe bottom so you can just take them in and out.

And no, the pedal will not be damaged unless you crash and hit it because you slipped off

I ride candies in tennis shoes occasionally. They're fine for going a slow couple of miles. I'll usually do it if I need to run an errand at lunch at work and don't feel like changing my shoes.

I feel unsafe riding with sneakers on clipless pedals, they are slippery. And the saddle feels way too high.

I've done hundreds of miles like this. Not ideal, but can be done. If anything, I've noticed the pedals can kinda chew up the bottom of a tennis shoe after a few hundred miles like this. The pedals showed zero damage, unless you're OCP about the paint wearing off.