Yes, you can use them with normal shoes, but as you predict, it isn't very comfortable, especially if your shoes have thin, flexible soles.
Also, there's a risk of your foot slipping off, particularly in the wet.
There are various options to temporarily convert clip pedals into ordinary flat ones.
Fly pedals
BBB BPD FeetRest pedal adaptors (SPD only)
bicycles.stackexchange.com/users/5337/James Bradbury
UPDATE:
I use Fly Pedals now when I want to convert my clipless pedals into regular pedals.
They are made of machined aluminum, light weight, and lock together so you can put them in your pocket.
They cost $50 plus cleats.
enter link description here
This company (started on Kickstarter) makes flat platforms with straps on top and cleats on the bottom.
They convert clipless pedals into toe clips.
A pair runs $55 without the actual cleats.
bicycles.stackexchange.com/users/5360/Gary E
There is nothing to stop you from getting on your bike in regular trainers instead of shoes with cleats.
you won't damage your pedals in any way as long as you don't have a rock lodged in the sole of your shoe.
The problem, as far as I can see it, is that it is just not very comfortable due to the small surface area and flexible sole of the shoe.
Your feet will feel the pressure localized into a very small area.
The other issue is grip.
There isn't a lot of traction since the body of the pedal wasn't designed with that in mind.
I do my own mechanics out if my garage and will occasionally hop on the bike I'm working on to make sure my repair or adjustment is working properly.
I have bikes with Ritchey Logic, Speedplay frogs, SpeedPlay Zeros, Shimano SPD and non-clipless pedals.
I don't go and put on the proper shoes just to ride down the street and back while I test an adjustment.
So, yes, you can ride without shoes with cleats but really only in a very limited way.
bicycles.stackexchange.com/users/19581/elessar007
Yes you can.
No it's not going to be comfortable.
You're more likely to slip off the pedals.
One alternative is to get double sided pedals where one side of the pedal has an SPD mount and the other side is a flat pedal.
I've been running Shimano M324 pedals on my commuter
so I can hop on with casual shoes or use my cycling shoes for more power on longer rides.
It's the worst of both worlds, so you'll find yourself trying to flip the pedal from time to time, but it's a doable option that gives you flexibility without having to swap pedals all the time.
Unfortunately, I don't know of any other type of shoe / cleat combination that works with dual sided pedals.
It seems that only shimano mountain SPD seems to offer this option.
If you're already sold on road pedals, then it's probably not going to work like this.
You might just want to invest in a set of flat pedals and swap your pedals when you want to ride without clipless shoes.
bicycles.stackexchange.com/users/2997/Benzo
I have used normal office shoes on look road pedals (albeit shoes with relatively thick soles)
This works fine for to/from work or lunchtime errands.
However I found that pedalling on the "underside" of the pedal was more comfortable in some thin-soled shoes.
Not ideal but workable.
Another option, try clipping a plastic cleat in the pedal, an older and worn-out one would be perfect.
Plus you can remove it when you want to ride with proper shoes.
I have look clipless on my road bike, but I put platforms back on my MTB for trips around town, mostly because our roads are still terrible, and the MTB has better brakes and more load-carrying capacity.
Cleats may not be best for you.
bicycles.stackexchange.com/users/19705/Criggie
Don’t wear sneakers over clip-less pedals!
Didn’t feel bad then
but when I woke up with ball area of the foot swollen with terrible pain.
Icing as I text.
I knew better too.
bicycles.stackexchange.com/users/49374/Frank D
Is it possible to use regular trainers/shoes (i.e., withou cleats) with clipless pedals?  A friend of mine mentioned he does this all the time. However, I cannot imagine how this would work. Surely the area of grip would be far too small to get any kind of purchase on the pedals.  We both have  Shimano PD-R540 SPD  pedals.
Can I use "normal" shoes/trainers with clipless pedals?
bicycles.stackexchange.com
<shoes><clipless><cleat><pedals-clipless>
bicycles.stackexchange.com/questions/30662
bicycles.stackexchange.com/users/18733/kaybee99
You can ride a bike in normal shoes without cleats but there are some disadvantages. First, the shoe will tend to slip off the pedal. Second, it will feel uncomfortable for the rider. It is, however, possible to convert your clip pedals into flat ones or you can get double-sided pedals. Alternatively, you could get a set of flat pedals to use when you don't want to wear cleats. You can use normal shoes without damaging your pedals, but the pedals will be uncomfortable and slip easily. A better solution might be to switch to double-sided or flat pedals, or use adapters that temporarily convert clipless pedals to flat pedals.
You can use normal shoes without damaging your pedals, but the pedals will be uncomfortable and slip easily. Fly Pedals are clips that temporarily convert clipless pedals to flat pedals. A better solution might be to switch to double-sided or flat pedals.