Yosi,
Arduinos are cute, and can be useful in certain situations. In the entertainment industry, we occasionally use an Arduino or a BASIC Stamp to do things like controlling RC hobby servos for animatronic effects. PLCs aren't so good at variable duty cycle PWM that RC servos need. Though opinions vary, I'm of the belief that they're mostly hobbyist-grade gear, and have no place where life and limb and other equipment is at stake.
Even if you subscribe to the "Arduinos are as good as PLCs" argument, consider this:
You and I need to control a project. You choose an Arduino and I choose a PLC. The project in question could use either successfully.
When your Arduino comes in the mail, you pull it from the box, grab a piece of perf board or a proto-shield, and begin soldering components to create an interface to the outside world. This is after you've breadboarded your circuit and made sure it will work. After some time, you now have a one-of-a-kind interface card that lets your Arduino control something larger than an LED. NOW you get down to programming, testing, and debugging. Provided you haven't made a soldering error, your project now works, but how do you mount it in your control cabinet? They don't come with DIN rail mounts, so you have to fab those too. What happens if you have a hardware failure? You're back to the lab bench to fire up the soldering iron again.
I have chosen a PLC. I pull it out of the box, clip it onto the DIN rail in my cabinet, feed it 24v power, land the I/O on the SCREW TERMINALS (no soldering required) and do my programming. While you're still soldering your custom I/O card, I'm done with everything and at the bar having a beer.
If I have a hardware failure (probably less likely in a designed and tested commercial product) I can have another one overnight. I reload the program, and I'm done with the repair.
Also beware of Cletus and Bubba. They're the neck-down hammer jockeys that will be tasked with repairing and maintaining the equipment into which you've placed an Arduino. How likely is it that your hand-soldered I/O board and open PCB will survive Cletus and Bubba's gentle ministrations?
BASIC Stamps, Arduinos, and PIC microcontrollers all have their place. They're a fantastic tool for teaching people, kids specifically, the basics of electronic and coding theory. A PLC is, at its most basic, nothing more than a microcontroller that's been armored against the real world.
As Peter said above, if you have thousands of copies of a thing to make, a microcontroller makes economic sense; you can design your control board to EXACTLY your specs. If you need one or a few, and need it to survive an industrial environment, be easy to maintain and repair/replace, than the PLC is your friend.
-rpoet