Are dummy security cameras worth using?

In a word, no. Experienced thieves can spot a dummy security camera from a mile away but here are a few more reasons.

Reasons not to use dummy security cameras

Dummy security cameras often look cheap, plastic, and fake so fool no one. Even the more expensive models can miss essential details like wiring or mounting systems.
The little blinking light which is meant to indicate a working camera is often a feature exclusive to dummy cameras. A real camera usually doesn't have a 'look at me I'm working' light.
Some fake cameras have a pan or tracking-like movement function. This feature works great if no one is actually in front of the camera but if an intruder walks by and the camera pans away from them they will probably just go about the business of breaking into your home undeterred.
To state the obvious, they don't actually DO anything. So if a break-in occurs you don't have any evidence or footage to help police find the thief.

Micron Security Camera — Security Camera - This is what a real one looks like

The few times it might be ok to fake it

Using fake security cameras in addition to a well set up, working surveillance system can sometimes increase the perception of constant monitoring and act as a deterrent.
If it's a dummy or nothing situation, go ahead and install the dummy cameras.

However, if you think your property could benefit from some form of security surveillance why not invest in the real thing, even if it's only one or two cameras, a small recording system, and self-monitoring it will be worth the investment in the long run.

Want to read more about using dummy security cameras? - Pros and cons of using fake security cameras