Putting foot out on clipless or no
 

I have noticed that a lot of people and even some pro racers put their foot out on corners or slippery parts of the trail just to make sure they don't fall over. I have always used clipless and was wondering if this is something worth doing with clipless(unclipping to do it and then clipping back in later) or whether it was really only a flat pedal technique.

If you have to unclip , do it -- better than falling. I rode on very sandy trails and having to unclip and re-engage 50 feet later is a fact of life

Your legs/feet are part of your balancing regardless of type of pedal. So, unclipping (or taking foot off flat pedal) to further increase your balance is important. It won’t make you less of a rider or something should you use to go clipless. In fact, it shows that you know what you are doing to keep yourself safe. The tricky part, especially for beginners on clipless pedals, is being able to unclip/clip fast enough on technical sections.

If I had to guess, I would say the pro's plan where they will unclip and clip back in to help improve their time through certain situations. However, I can't think of a good example.

Typically, “dropping a foot” in a corner is not done to actually touch the ground with your foot but to change you balance on the bike. Putting a foot out to catch yourself “just in case” is usually a really bad idea.

It is useful sometime, but whether and when it is worth doing really comes down to how fast you are at getting back in the pedal, and what it is like riding before you do.

I use Frogs and can be clipped back in by the end of the next pedal stoke, so I drop a foot in a corner whenever it feels good.

I usually just fall over and take my bruises as a sign of pride that I NEVER unclip. it's never worth it to let other riders know that you're not hardcore.

Keep your feet on the pedals, clipped in or not. I ride motorcycles off road a lot, and if you have to dab a foot, you have already lost. No sense in getting your lower leg ran over in the process.

Really? Very few people are racing all the time so there's no 'losing'. Nor is there any risk of getting run over. For the OP...do you really need to ask about this? Just try it and see, if it works for you it works. If it doesn't, don't do it.