Hooked on conventional reels.

[JohnR]

Several things:

It doesn't have a drag system and no anti-reverse.

Your reel has a drag or it couldn't function...

Any surfcasters use conventionals for plugging? What reels do you use?

Your plugging reels are the the regular reels in most cases but maybe drop down a size with light plugs...

When I was casting, I noticed some times, my sinker got snagged. When I pulled hard and popped it out of the ground, the sinker came flying towards me. The problem was the line had slack in it.

Your sinker stuck because you burried it in the ground, something about the ballistic arc of a piece of lead. Your line had slack because you didn't "thumb it"

I know the sinker in the ground won't happen in the surf but what if a wave pushes your plug in?

The wave wont "push" your plug in unless your plugging bait dunking style which wouldn't make a whole lot of sense. In order for your plug to work you need to work the plug. You're not going to allow so much slack in the line in the first place because you can't control it. Or set up on a fish in the unlikely event it gets whacked when you're just letting it do nothing (it does happen on occasion when a plug is just sitting). The line was really loose because you need to prevent that line over run with your thumb. If you didn't slightly burn the thumb on your soft, unworked, non-callous hands then you weren't doing it right...

When do you prefer conventional over spinning gear?

Almost all of the time. Real light stuff might go to a spinner...