I’ve worked in a hell of a lot of clubs who have delivery people, sales people, and others are coming and going all day. Often with no one keeping an eye on FOH and DJ Booths. I don’t care how big something is, it only takes one person to realize there is something there for easy taking. (Disconnect a few cables and viola! Gone.)
Mounting equipment absolutely stops the grab-and-run. If it takes someone a couple minutes to unbolt something, then their a lot less likely to go for it. Additionally there are security bolts available, so if I’m that concerned that a location may have people with screwdrivers going for the equipment, then those will generally stop them in their path.
Again what we’re going for is to stop the knob-rattlers/opportunity takers. If someone is really intent on taking something, you’re right, they will figure out a way.
As a commercial AV system designer, I’ve seen it all. DJs steal equipment. Vindictive, fired employees steal equipment. Employees on their last day steal equipment. People breaking into clubs after hours steal equipment. It’s all fair game when it isn’t bolted down.