This can be a really confusing area for dog and cat owners because a lot of places now sell lasers as toys for animals. But the evidence from vets and dog trainers and behaviourists points to this being a very
bad idea -- and probably worse for cavaliers than some other breeds as they are already inclined towards more reports of obsessive compulsive behaviour (like flycatching and tailchasing) than many other breeds.
This is a good overview article by a dog trainer, and I'd agree with what he says, while adding that some lasers could harm an animal's eyes too -- it depends on the strength of the light beam. Consider they have been used to blind pilots landing planes though and I think we can agree getting a laser anywhere near an animal's eyes is a risk not worth taking, ever:
On a further note, I have read several personal reports of people with dogs, many of them cavaliers, who have ended up with serious obsessive-compulsive behaviours after chasing lasers. The questions comes up regularly on some of the email lists and boards.
There are vet studies that draw a direct connection between lasers and obsessive-compulsive disorders.
"Dr. Alice Moon-Fanelli is a behavior geneticist and consultant on companion animal behavior at Tufts University School of Veterinary Medicine. She recently told me, "It is well known in the academic behavior community that some dogs at risk for developing compulsive behavior can be triggered by laser lights to become compulsive light chasers."
"Kathy Diamond, author of "Therapy Dogs: Training Your Dog to Reach Others," said in a recent article in VeterinaryPartner.com, "Laser lights and flashlight games need special mention. Don't! These games commonly lead to obsessive behaviors. A laser light can also damage a dog's eyes. When an obsession with light-chasing becomes full-blown, the dog may require lifelong medication to control ceaseless searching for any beam of light or shadow to follow."