Note that the linked article is from 2010. I don't have any hard statistics on how common false alerts are, but I do have some anecdotal info. First off, we need to distinguish between unintentional accidental activation of the device, and intentional but unnecessary activation (like the skier in the article). Both happen.

The devices can sometimes be activated by accident. The early InReach units manufactured by DeLorme are somewhat notorious for this. It seems the safety lock was poorly designed and the SOS button could fairly easily be triggered by being jostled around in your pack. There were quite a few cases of this. It happened to one friend of mine, fortunately they quickly realized what happened and canceled the SOS before a rescue was launched. I have personal knowledge of another case where an early DeLorme InReach was activated, and a rescue was launched. However, the search managers noticed that the unit was still moving, in a direction away from safety. Further investigation revealed it was a false alarm, before any SAR teams actually left the staging area. Note that when you trigger an SOS on an InReach, the first thing that happens is that the global alert center tries to send you a message to verify that you are in need of rescue. Of course if the unit is buried in your pack, you might not realize they're trying to contact you! Also note that Garmin has fixed the safety lock on the newer InReach units, and accidental activation doesn't seem to be a problem with these units built by Garmin.

ELTs carried by aircraft and EPIRBs carried on boats also have problems with accidental activation. The ELTs are designed to be activated by G forces when a plane crashes. Occasionally, a hard landing (not a crash) will set one off. Many boats carry EPIRBs in a special bracket designed such that if the boat sinks the EPIRB will float free and activate. Sometimes a boat will take an exceptionally big wave which washes the EPIRB free. Or someone inadvertently knocks it free from it's bracket and it activates. One funny incident I heard about was some time back there was a case in SE Alaska when an EPIRB was activated. It was eventually traced to a barge loaded with garbage, being pulled by a tug. It seems that folks were dismantling an old boat, and tossed the EPIRB into the trash along with other boat parts. Trash in this town gets sent by barge for disposal down in Washington. The EPIRB somehow got turned on in the trash pile on the barge!

I was once at a talk by an officer from the Alaska Rescue Coordination Center. He said when circumstances suggest an accidental activation they will generally spend some time trying to contact the vessel, aircraft, or whomever the device is registered to, in order to verify that it is really in trouble, before launching a rescue. A good reason to keep your registration up to date, whether for ELT, EPIRB, PLB, InReach or SPOT!

The other issue, of course, is when people activate a PLB, InReach or whatever when they aren't really in trouble. Like the skier in the story linked in the OP. Some folks don't seem to realize that these units are for true emergencies. Others may feel like they are in an emergency when in reality they probably could suck it up and self rescue. There have been a few well publicized incidents like this. There was the famous "Yuppie 911" incident in the Grand Canyon some years back. I don't think there are that many truly egregious cases, but it does happen now and then. Like anything else, we do need to educate people on when and how they should use these devices.

Don't use it unless you really need it, but if you need it, don't hesitate to activate it. And keep your registration up to date!