I was told that it's a Bad Idea (especially for root ) to Add To the Variable $PATH in unix the ":." (dot),
In order to execute programs in my current directory without typing ./program
For example: PATH=$PATH:$HOME/bin:.
Does someone know why is it a Bad Idea?

Simply because you may have something in your CWD that is the same name as a Unix command. For example (an extreme case I know), imagine you had this as a script called ls.
now imagine running that as root, from roots' $HOME

In addition to avoiding accidents like reborg talked about, it is also a security issue. A virus, trojan, or hacker can insert nasty code with the same name as a common Unix command anywhere and hope root executes it by accident if "." at the start of root's path.

For example, if I break into your box as a normal non-root user, I could create a file named "cd" in some directory I have write access to. What if my script has one simple command:

rm -rf $1

Now when root tries to cd to anywhere from the directory I put my script in, it wipes out that directory instead. Alternately, you could have your script email vital files to the attacker, change data, or any other bad thing you can imagine. If you can trick root into running it for you, he's hosed.

It's basically the same thing as reborg said I guess. But you may have thought "I'd never name a script like that and cause a problem." My example above is why you still shouldn't have . in your path - you might not do something dumb yourself, but having . in the path makes it easier for the bad guys to get you.